How to calculate the molecular weight of a substance. How to find the mass of a molecule

Instruction

If you carefully consider the table of Dmitry Ivanovich Mendeleev, you can see that it looks like a multi-apartment multi-storey building, in which there are "residents" - elements. Each of them has a surname () and chemical. Moreover, each of the elements lives in its own apartment, and therefore has. This information is presented in all cells of the table.

However, there is another figure, at first glance, completely incomprehensible. Moreover, it is indicated with several values after the decimal point, which is done for greater accuracy. It is this number that you need to pay attention to, because this is the relative atomic mass. Moreover, this is a constant value that does not need to be memorized and can be found in the table. By the way, even at the exam according to D.I. Mendeleev is reference material, available for use, and each one is in an individual package - KIM.

The molecular mass, or rather the relative substance, denoted by the letters (Mr) is made up of the relative atomic masses (Ar) that form the molecule, the elements. The relative atomic mass is just the one mysterious figure, which stands in each cell of the table. For calculations, these values must be rounded up to a whole number. The only exception is the chlorine atom, whose relative atomic mass is 35.5. This characteristic has no units of measurement.

Example 1. Find the molecular mass(KOH)
The potassium hydroxide molecule consists of one potassium atom (K), one oxygen atom (O), and one hydrogen atom (H). Therefore, we find:
Mr (KOH) \u003d Ar (K) + Ar (O) + Ar (H)

Hence: Mr (KOH) = 39 + 16 + 1 = 56

Example 2. Find the molecular mass sulfuric acid (H2SO4 ash-two-es-o-four)
The sulfuric acid molecule consists of two hydrogen atoms (H), one sulfur atom (S), and four oxygen atoms (O). Therefore, we find:
Mr(H2SO4) = 2Ar(H) + Ar(S) + 4Ar(O)
According to the table D.I. Mendeleev, we find the values of the relative atomic masses of the elements:
Ar (K) = 39, Ar (O) = 16, Ar (H) = 1
Hence: Mr (H2SO4) = 2 x 2 + 32 + 4 x 16 = 98

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note

When calculating, multiplication or division is performed first, and only then addition or subtraction is performed.

Helpful advice

When determining the relative atomic mass, round off the values that are in the D.I. table. Mendeleev to an integer

Sources:

how to calculate molecular weight
Molecular weight definition

To find the molecular mass, find the molar mass substances in grams per mole, since these quantities are numerically equal. Or search mass particles of a molecule in atomic mass units, add their values and get the molecular mass. To find the molecular weight of a gas, you can use the Clapeyron-Mendeleev equation.

You will need

For calculations, you will need the periodic table of Mendeleev, scales, thermometer, pressure gauge.

Instruction

Calculation using the periodic table. Determine chemical formula the substance under study. Find in the periodic table chemical elements that make up the molecule. In the corresponding cells, find their atomic mass. If the table represents a fractional number, round it up to a whole number. If the same element occurs several times in a molecule, multiply it mass for the number of entries. Add up all the atoms. The result is substances.

Calculation of molecular weight when converted from grams. If the mass of one molecule is given in grams, multiply it by Avogadro's constant, which is 6.022 10^(23) 1/mol. The result will be the substance in grams per mole. Its numerical value coincides with the molecular weight in atomic mass units.

Calculation of the molecular weight of an arbitrary gas. Take a cylinder of known volume measured in cubic meters, pump out air from it and weigh it on a balance. Then, pump gas into it, molecular mass which needs to be determined. find again mass balloon. The difference between the gas cylinder and the empty cylinder will be equal to the mass of gas, carry out in grams. Measure the pressure with a pressure gauge (at) and the temperature with a thermometer by converting it to . To do this, add the number 273 to the degrees Celsius obtained as a result of the measurement. To find the molar mass gas, his mass multiply by the temperature and the number 8.31 (universal gas constant). The result obtained is successively divided by the value of the gas pressure and its volume M = m 8.31 T / (P V). This indicator, expressed in grams per mole, is numerically the molecular weight of the gas, expressed in atomic mass units.

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molecular weight calculation

The relative molecular weight of a substance (or simply the molecular weight) is the ratio of the mass value of a given substance to 1/12 of the mass of one carbon atom (C). Find the relative molecular weight mass very easy.

You will need

Periodic table and molecular weight table

Instruction

The relative of a substance is the sum of its atomic masses. In order to learn the atomic mass one way or another, just look at the periodic table. It can be found on the cover of any software, or purchased separately at a bookstore. For a pocket version, or an A4 sheet is quite suitable. Any modern chemistry is equipped with a full-scale wall periodic table.

Having learned the nuclear mass element, you can begin to calculate the molecular weight of the substance. This is easiest to show with an example:
It is required to calculate the molecular mass water (H2O). It can be seen from the molecular formula that a water molecule consists of two H atoms and one O atom. Therefore, the calculation of the molecular weight of water can be reduced to the action:
1.008*2 + 16 = 18.016

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note

Atomic mass as a concept appeared in 1803, thanks to the work of the then famous chemist John Dalton. In those days, the mass of any atom was compared with the mass of a hydrogen atom. This concept was further developed in the works of another chemist, Berzelius, in 1818, when he proposed using an oxygen atom instead of a hydrogen atom. Since 1961, chemists of all countries have taken as a unit of atomic mass the mass of 1/16 of an oxygen atom, or the mass of 1/12 of a carbon atom. The latter is just indicated in the periodic table of chemical elements.

Helpful advice

When using the periodic table in the form in which it is presented in most textbooks on chemistry and other reference books, it must be understood that this table is a shortened version of the original periodic table. In its most complete version, a separate line is devoted to each chemical element.

The molecular weight of a substance refers to the total atomic mass of all the chemical elements that are part of that substance. To calculate the molecular mass substances, no special effort is required.

You will need

periodic table.

Instruction

Now you need to take a closer look at any of the elements in this table. Under the name of any of the elements indicated in the table there is a numerical value. It is it and the atomic mass of this element.

Now it is worth analyzing a few examples of calculating the molecular weight, based on the fact that atomic masses are now known. For example, you can calculate the molecular weight of a substance such as water (H2O). A water molecule contains one oxygen atom (O) and two hydrogens (H). Then, having found the atomic masses of hydrogen and oxygen from the periodic table, we can begin to calculate the molecular mass: 2 * 1.0008 (after all, there are two hydrogens) + 15.999 = 18.0006 amu (atomic mass units).

Another . The next substance, molecular mass which can be calculated, let it be ordinary table salt (NaCl). As can be seen from the molecular formula, the molecule table salt contains one Na atom and one Cl atom. In this case, it is considered as follows: 22.99 + 35.453 = 58.443 a.m.u.

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note

It should be noted that the atomic masses of isotopes various substances differ from the atomic masses in the periodic table. This is due to the fact that the number of neutrons in the nucleus of an atom and inside an isotope of the same substance is different, so the atomic masses also differ markedly. Therefore, isotopes of various elements are usually denoted by the letter of the given element, while adding its mass number in the upper left corner. An example of an isotope is deuterium ("heavy hydrogen"), the atomic mass of which is not one, like an ordinary atom, but two.

Molar is weight one mole of a substance, that is, such an amount of it that contains as many atoms as 12 grams of carbon. In another way, such a quantity is called the number (or constant) of Avogadro, in honor of the Italian scientist who first put forward the hypothesis. According to it, equal volumes of ideal gases (at the same temperatures and pressures) must contain the same number of molecules.

It must be firmly remembered that one mole of any substance is approximately 6.022 * 1023 molecules (either atoms or ions) of this substance. Therefore, any amount of any substance can be represented by elementary calculations in the form of a certain number of moles. And why was the mole introduced at all? To facilitate calculations. After all, the number of elementary (molecules, atoms, ions) even in the smallest sample of a substance is simply colossal! Agree, it is much more convenient to express the amount of substances in moles than in huge zeros with endless rows! Molar weight substance is determined by adding the molar masses of all the elements included in it, taking into account the indices. For example, you need to determine the molar mass of anhydrous sodium sulfate. First of all, write down its chemical formula: Na2SO4. Do the calculations: 23*2 + 32 + 16*4 = 142 grams/mol. This will be the molar weight this salt. And if you need to determine the molar mass a simple substance? The rule is exactly the same. For example, molar weight oxygen O2 \u003d 16 * 2 \u003d 32 grams / mol, molar weight N2 \u003d 14 * 2 \u003d 28 grams / mol, etc. It is even easier to determine the molar mass, the molecule of which consists of one atom. For example, molar weight sodium is 23 / mol, silver - 108 grams / mol, etc. Of course, rounded values are used here to simplify calculations. If greater accuracy, it is necessary for the same sodium to consider its relative atomic mass equal not to 23, but to 22.98. It must also be remembered that the value of the molar mass of a substance depends on its quantitative and quality composition. So different substances with the same number of moles, they have different molar masses.

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Tip 6: How to Determine Relative Molecular Weight

The relative molecular weight of a substance is a value showing how many times the mass of one molecule of a given substance is greater than 1/12 of the mass of the carbon isotope. In another way, it can be called simply molecular weight. How can you find the relative molecular mass?

You will need

Periodic table.

Instruction

All you need for this is the periodic table and the elementary ability to make calculations. After all, the relative molecular mass is the sum of the atomic masses of the elements that make up the one you are interested in. Of course, taking into account the indices of each element. The atomic mass of each element is listed in the Periodic Table along with other important information, and with very high accuracy. For these purposes, rounded values \u200b\u200bare quite suitable.

Now take the periodic table and determine the atomic masses of each element included in its composition. There are three such elements: , sulfur, . Atomic mass (H) \u003d 1, atomic mass of sulfur (S) \u003d 32, atomic mass of oxygen (O) \u003d 16. Given the indices, sum up: 2 + 32 + 64 \u003d 98. This is the relative molecular weight of sulfuric acid. note that we are talking about the approximate, rounded result. If, for some reason, accuracy is required, then it will be necessary to take into account that the atomic mass of sulfur is not exactly 32, but 32.06, hydrogen is not exactly 1, but 1.008, etc.

note

If the periodic table is not at hand, find out the relative molecular weight of a particular substance using reference books on chemistry.

Helpful advice

The mass of a substance in grams, which is numerically equal to its relative molecular weight, is called a mole.

The relative molecular weight of a substance shows how many times a molecule of a given substance is heavier than 1/12 of an atom of pure carbon. It can be found if its chemical formula is known, using Mendeleev's periodic table of elements. Otherwise, use other methods to find the molecular weight, given that it is numerically equal to the molar mass of the substance, expressed in grams per mole.

You will need

- periodic table of chemical elements;
- hermetic container;
- scales;
- manometer;
- thermometer.

Instruction

If a substance is known, determine its molecular weight using Mendeleev's periodic table of chemical elements. To do this, identify the elements that are in the formula of the substance. Then, find their relative atomic masses, which are recorded in the table. If the atomic mass in the table is a fractional number, round it up to the nearest whole number. If it contains several atoms of the given element, multiply the mass of one atom by their number. Add up the resulting atomic masses and get the relative molecular mass of the substance.

For example, to find the molecular weight of sulfuric H2SO4, find the relative atomic masses of the elements that are included in the formula, respectively, of sulfur and oxygen Ar(H)=1, Ar(S)=32, Ar(O)=16. Given that there are 2 atoms of hydrogen in a molecule, and 4 atoms of oxygen, calculate the molecular weight of the substance Mr(H2SO4)=2 1+32+4∙16=98 atomic mass units.

In the event that the amount of the substance in moles ν and the mass of the substance m, expressed in grams, are known, determine its molar mass; for this, divide the mass by the amount of the substance M=m/ν. It will be numerically equal to its relative molecular weight.

If you know the number of molecules of a substance N, the known mass m, find its molar mass. It will be equal to the molecular weight by finding the ratio of the mass in grams to the number of molecules of the substance in this mass, and multiply the result by the Avogadro constant NA = 6.022 ^ 23 1 / mol (M = m ∙ N / NA).

To find the molecular weight of an unknown gas, find its mass in a known sealed volume. To do this, pump the gas out of it by creating a vacuum there. Weigh. Then pump the gas back in and find its mass again. The difference between the masses of the empty and filled cylinder will be equal to the mass of the gas. Measure the pressure inside the cylinder with a pressure gauge in Pascals, and in Kelvins. To do this, measure the ambient temperature, it will be equal to the inside of the cylinder in degrees Celsius, to convert it to Kelvin, add 273 to the resulting value.

Determine the molar mass of a gas by finding the product of the temperature T, the mass of the gas m, and the universal gas constant R (8.31). Divide the resulting number by the values of pressure P and volume V, measured in m³ (M \u003d m 8.31 T / (P V)). This number will correspond to the molecular weight of the gas under study.

Hydrogen is the first element in the periodic table and the most common in the universe, since it is from it that stars are mainly composed. It is part of the vital substance for biological life - water. Hydrogen, like any other chemical element, has specific characteristics, including its molar mass.

Instruction

Remember molar mass? This is the mass of one mole, that is, such an amount of it, in which there is approximately 6.022 * 10 ^ 23 elementary particles substances (atoms, molecules, ions). This number is called "Avogadro's number", and is named after the famous scientist Amedeo Avogadro. The molar mass of a substance numerically coincides with its molecular mass, but has a different dimension: not atomic mass units (amu), but gram / mol. Knowing this, determine the molar mass hydrogen as easy as pie.

What has a molecule hydrogen? It is diatomic, with the formula H2. Immediately: a molecule is considered, consisting of two atoms of the lightest and most common hydrogen isotope, protium, and not of the heavier one

The molecule of a substance is at the same time the smallest possible portion of it, and therefore it is precisely its properties that are decisive for the substance as a whole. This particle belongs to the microcosm, therefore, it is not possible to consider, let alone weigh it. But the mass of one molecule can be calculated.

You will need

- the concept of the structure of the molecule and the atom;
- calculator.

Instruction

If the chemical formula is known substances, determine its molar mass. To do this, determine the atoms that make up the molecule, and find their relative atomic masses in the periodic system of chemical elements. If one atom occurs n times in a molecule, multiply it mass for this number. Then add the found values and get the molecular mass given substances, which is equal to its molar mass in g/mol. Find mass one molecules by dividing the molar mass substances M to the Avogadro constant NА=6.022 10^23 1/mol, m0=M/NA.

Example Find mass one molecules water. The water molecule (H2O) consists of two hydrogen atoms and one oxygen atom. The relative atomic mass of hydrogen is 1, for two atoms we get the number 2, and the relative atomic mass of oxygen is 16. Then the molar mass of water will be 2+16=18 g/mol. Determine mass one molecules: m0=18/(6.022^23) 3 10^(-23)

mass molecules can be calculated if the number of molecules in a given substance is known. To do this, divide the total mass substances m by the number of particles N (m0=m/N). For example, if it is known that in 240 g substances contains 6 10^24 molecules, then the mass of one molecules will be m0=240/(6 10^24)=4 10^(-23)

Determine mass one molecules substances with sufficient accuracy, knowing the number of protons and neutrons that are part of its nuclei of the atoms of which it consists. Mass electron shell and the mass defect in this case should be neglected. Take the mass of the proton and neutron equal to 1.67 10^(-24) g. For example, if it is known that a molecule consists of two oxygen atoms, what is its mass? The nucleus of an oxygen atom has 8 protons and 8 neutrons. The total number of nucleons is 8+8=16. Then the mass of the atom is 16 1.67 10^(-24)=2.672 10^(-23) g. Since the molecule consists of two atoms, its mass is 2 2.672 10^(-23)=5.344 10^(-23 ) G.

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The mass of 1 mol of a substance is called its molar mass and is denoted by the letter M. The units of measurement of the molar mass are g / mol. How this value is calculated depends on the given conditions. You will need- periodic system chemical elements D.I.…

Quantity molecules in matter, it is virtually unthinkable to measure by ordinary methods. This is due to the fact that the molecule of the substance is too small to see it. Consequently, the number of molecules in a given mass of a substance is calculated using special formulas.

You will need

- periodic table of chemical elements;
- scales;
- calculator.

Instruction

1. Knowing such a quantity as the number of substance?, discover the number molecules in him. To do this, multiply the number of a substance, measured in moles, by the continuous Avogadro (NA = 6.022? 10 ^ 23 1 / mol), which is equal to the number molecules in 1 mole of substance N=?/NA. Let's say if there is 1.2 moles of table salt, then it contains N=1.2?6.022?10^23?7.2?10^23 molecules .

2. If the chemical formula of a substance is known, with the support of the periodic table of elements, find its molar mass. To do this, in the table, find the relative nuclear masses of the atoms that make up molecules uh, and fold them up. As a result, you get a relative molecules the mass of a substance, which is numerically equal to its molar mass in grams per mole. After that, on the scales, measure the mass of the test substance in grams. To discover the number molecules in a substance, multiply the mass of the substance m by the continuous Avogadro (NA=6.022?10^23 1/mol) and divide the result by the molar mass M (N=m? NA/M).

3. Example Define a number molecules, which is contained in 147 g of sulfuric acid. Find the molar mass of sulfuric acid. Her molecules a consists of 2 hydrogen atoms, one sulfur atom and 4 oxygen atoms. Their nuclear masses are 1, 32 and 16. The relative molecules the yar mass is equal to 2?1+32+4?16=98. It is equal to the molar mass, therefore M=98 g/mol. Then the number molecules contained in 147 g of sulfuric acid will be equal to N=147?6.022?10^23/98?9?10^23 molecules .

4. To discover the number molecules gas under typical conditions at a temperature of 0? C and a pressure of 760 mm Hg. column, find its volume. To do this, measure or calculate the volume of the container V in which it is located in liters. To discover the number molecules gas, divide this volume by 22.4 liters (the volume of one mole of gas under typical conditions), and multiply by the Avogadro number (NA \u003d 6.022? 10 ^ 23 1 / mol) N \u003d V? NA/22.4.

A. Avogadro in 1811, at the very beginning of the formation of nuclear theory, made the assumption that an equal number of perfect gases at identical pressure and temperature contain an identical number of molecules. Later this assumption was confirmed and became a necessary consequence for kinetic theory. Now this theory is called Avogadro.

Instruction

1. Avogadro's law: One mole of ideally any gas, if the temperature and pressure are identical, will occupy the same volume of molecules. Under typical conditions, this volume is equal to - 22.41383 liters. This value determines the molar volume of the gas.

2. The Avogadro constant shows the number of atoms or molecules that are contained in one mole of a substance. The number of molecules, provided that the system is one-component, and the molecules or atoms of the same type contained in it, can be detected by a special formula

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A molecule is an electrically neutral particle that owns all chemical properties characteristic of that particular substance. Including gases: oxygen, nitrogen, chlorine, etc. How is it possible to determine the number of gas molecules?

Instruction

1. If you need to count how many oxygen molecules are contained in 320 grams of this gas under typical conditions, before each one, determine how many moles of oxygen are included in this number. According to the periodic table, it is possible to see that the rounded nuclear mass of oxygen is 16 nuclear units. From the fact that the oxygen molecule is diatomic, the mass of the molecule will be 32 nuclear units. Therefore, the number of moles is 320/32 = 10.

2. Further help is the universal Avogadro number, named after the scientist who suggested that equal volumes of perfect gases under continuous conditions contain identical numbers of molecules. It is denoted by the symbol N (A) and is huge - approximately 6.022 * 10 (23). Multiply this number by the calculated number of moles of oxygen and you will find out that the desired number of molecules in 320 grams of oxygen is 6.022 * 10 (24).

3. And if you know the pressure of oxygen, as well as the volume occupied by it, and the temperature? How to calculate the number of its molecules with such data? And there is nothing difficult here. You just need to write down the universal Mendeleev-Clapeyron equation for perfect gases: PV = RTM / m Where P is the gas pressure in pascals, V is its volume in cubic meters, R is the universal gas continuous, M is the mass of the gas, and m is its molar mass .

4. Transforming this equation slightly, you get: M = PVm/RT

5. From the fact that you have all the necessary data (pressure, volume, temperature are set initially, R \u003d 8.31, and the molar mass of oxygen \u003d 32 grams / mol), you will simply find the mass of gas at a given volume, pressure and temperature. And then the problem is solved correctly in the same way as in the above example: N(A)M/m. By doing the calculations, you will find out how many oxygen molecules are contained under given conditions.

6. It is allowed to simplify the solution even more, from the fact that in the resulting fraction N(A)PVm/RTm the molar masses are reduced, and it remains: N(A)PV/RT. By substituting the values known to you into the formula, you will get the result.

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Helpful advice
Not a single real gas (including oxygen), of course, is irreproachable, therefore the Mendeleev-Clapeyron equation can be used for calculations only under conditions that are not very different from typical ones.

A molecule is so tiny in size that the number of molecules even in a tiny grain or drop of some substance will easily be colossal. It cannot be measured with the help of ordinary methods of calculation.

What is "mole" and how to use it to find the number of molecules in a substance

To determine how many molecules are in one or another number of a substance, the representation "mole" is used. A mole is the number of a substance that contains 6.022*10^23 of its molecules (either atoms or ions). This huge value is called the "continuous Avogadro", it is named after the famous Italian scientist. The value is denoted NA. With the support of continuous Avogadro, it is very easy to determine how many molecules are contained in any number of moles of any substance. Let's say 1.5 moles contains 1.5*NA = 9.033*10^23 molecules. In cases where a very high measurement accuracy is required, it is necessary to use the value of the Avogadro number with a huge number of decimal places after the decimal point. Especially its total value is: 6.022 141 29 (27) * 10 ^ 23.

How is it possible to detect the number of moles of a substance

Determining how many moles are contained in a certain number of substances is very easy. To do this, you only need to have the exact formula of the substance and the periodic table at hand. Imagine you have 116 grams of regular table salt. You need to determine how many moles are contained in such a number (and, accordingly, how many molecules are there)? Before each, remember the chemical formula of table salt. It looks like this: NaCl. The molecule of this substance consists of 2 atoms (or rather, ions): sodium and chlorine. What is its molecular weight? It is made up of nuclear masses of elements. With the support of the periodic table, you know that the nuclear mass of sodium is approximately 23, and the nuclear mass of chlorine is 35. Consequently, the molecular mass of this substance is 23 + 35 = 58. The mass is measured in nuclear mass units, where the lightest atom is taken as the standard - hydrogen. And knowing the molecular weight of a substance, you will be able to determine its molar mass (that is, the mass of one mole) here. The fact is that numerically the molecular and molar masses completely coincide, they only have different units of measurement. If the molecular weight is measured in nuclear units, then the molar mass is in grams. Therefore, 1 mole of table salt weighs approximately 58 grams. And you, according to the conditions of the problem, have 116 grams of table salt, that is, 116/58 = 2 moles. Multiplying 2 by continuous Avogadro, you will find that there are approximately 12.044*10^23 molecules in 116 grams of sodium chloride, or approximately 1.2044*10^24.

A molecule is the smallest unit of a substance that still retains the characteristics of that substance. It has a very small size, it cannot be seen with the naked eye or weighed. How to calculate the mass of a molecule?

Mole and atomic weight of a molecule

To calculate the mass of a molecule, a unit called a mole and a measurement called atomic weight are used. A mole is a quantity equal to the Avogadro number, which is approximately 6.022 x 10 ^ 23. Atomic weight is the weight of one atom of a substance in units of atomic mass. The weight of one mole of an element is equal to the atomic weight of the element. Knowing this, it is possible to derive the mass of each molecule from its chemical formula and the atomic weights of its elements, as described in the Periodic Table.

Set the chemical formula of the molecule

Find the atomic weight of each atom in the molecule. You can find this information in the Periodic Table; it's usually decimal number, above or below the element symbol.
For example, the atomic mass of hydrogen is 1.0079 and the atomic mass of oxygen is 15.999. One mole of each element weighs the same amount in grams.
Further, based on the formula already known to us, we calculate the sum of all atoms of the molecule.
In particular, a water molecule contains two hydrogen atoms and one oxygen atom. In this case, the molar mass of water is 1.0079 + 1.0079 + 15.999, or 18.0148 g per mol (g/mol).
Divide the sum of the molecules by the mole or Avogadro's number (6.022 x 10^23).
For example, 18.0148 / 6.022 x 10^23 = 2.991 x 10^23. Thus, one water molecule weighs 2.991 x 10^23 g.

As you can see from the example above, using Periodic table Mendeleev, you can get all the necessary indicators to calculate the mass of molecules of various substances.

Quantity molecules in substance almost impossible to measure by conventional methods. This is due to the fact that the molecule of the substance is too small to be seen. Therefore, the number of molecules in a given mass of a substance is calculated using special formulas.

You will need

- periodic table of chemical elements;
- scales;
- calculator.

Instruction

Knowing such a quantity as the amount of substance ?, find the number molecules in him. To do this, multiply the amount of a substance, measured in moles, by the Avogadro constant (NA \u003d 6.022 10 ^ 23 1 / mol), which is equal to the number molecules in 1 mole of substance N=?/NA. For example, if there is 1.2 mol of table salt, then it contains N=1.2 6.022 10^23?7.2 10^23 molecules.

If you know the chemical formula of a substance, use the periodic table of elements to find its molar mass. To do this, in the table, find the relative atomic masses of the atoms that make up molecules uh, and fold them up. The result is a relative molecules the mass of a substance, which is numerically equal to its molar mass in grams per mole. Then, on the scales, measure the mass of the test substance in grams. To find the quantity molecules in substance, multiply the mass of the substance m by Avogadro's constant (NA=6.022 10^23 1/mol) and divide the result by the molar mass M (N=m NA/M).

Example Determine Quantity molecules, which is contained in 147 g of sulfuric acid. Find the molar mass of sulfuric acid. Her molecules a consists of 2 hydrogen atoms, one sulfur atom and 4 oxygen atoms. Their atomic masses are 1, 32 and 16. Relative molecules the yar mass is equal to 2 1+32+4 16=98. It is equal to the molar mass, so M=98 g/mol. Then the amount molecules contained in 147 g of sulfuric acid will be equal to N=147 6.022 10^23/98?9 10^23 molecules.

To find the quantity molecules gas under normal conditions at a temperature of 0? C and a pressure of 760 mm Hg. column, find its volume. To do this, measure or calculate the volume of the container V in which it is located in liters. To find the quantity molecules gas, divide this volume by 22.4 liters (the volume of one mole of gas under normal conditions), and multiply by the Avogadro number (NA \u003d 6.022 10 ^ 23 1 / mol) N \u003d V NA / 22.4.

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The molecule of a substance is at the same time the smallest possible portion of it, and therefore it is precisely its properties that are decisive for the substance as a whole. This particle belongs to the microcosm, therefore it is impossible to consider, let alone weigh it...

The molecular formula of a substance shows exactly which chemical elements and in what quantity are part of this substance. In practice, it is determined in various ways, both experimentally, using quantitative and qualitative methods ...

The mass of a substance is found using a device called a balance. It is also possible to calculate the mass of a body if the amount of a substance and its molar mass or its density and volume are known. Quantity pure substance you can find by its mass or ...

In order to find the molar mass of a substance, determine its chemical formula and use the periodic table to calculate its molecular weight. It is numerically equal to the molar mass of the substance in grams per mole. If the mass of one is known ...

To find the volume of one mole of a substance in a solid or liquid state, find its molar mass and divide by the density. One mole of any gas under normal conditions has a volume of 22.4 liters. In the event that conditions change, calculate the volume of one ...

To do this, you need to add the masses of all the atoms in this molecule.

Example 1. In the water molecule H 2 O 2 hydrogen atoms and 1 oxygen atom. The atomic mass of hydrogen \u003d 1, and oxygen \u003d 16. Therefore, the molecular mass of water is 1 + 1 + 16 \u003d 18 atomic mass units, and the molar mass of water \u003d 18 g / mol.

Example 2. In a molecule of sulfuric acid H 2 SO 4 there are 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms. Therefore, the molecular weight of this substance will be 1 2 + 32 + 4 16 \u003d 98 amu, and the molar mass will be 98 g / mol.

Example 3. In a molecule of aluminum sulfate Al 2 (SO 4) 3 2 aluminum atoms, 3 sulfur atoms and 12 oxygen atoms. The molecular weight of this substance is 27 2 + 32 3 + 16 12 = 342 amu, and the molar mass is 342 g / mol.

Mole, molar mass

Molar mass is the ratio of the mass of a substance to the amount of a substance, i.e. M(x) = m(x)/n(x), (1)

where M(x) is the molar mass of substance X, m(x) is the mass of substance X, n(x) is the amount of substance X.

The SI unit for molar mass is kg/mol, but the unit g/mol is commonly used. Mass unit - g, kg.

The SI unit for the amount of a substance is the mole.

A mole is such an amount of a substance that contains 6.02 10 23 molecules of this substance.

Any problem in chemistry is solved through the amount of substance. You need to remember the basic formulas:

n(x) =m(x)/ M(x)

or general formula: n(x) =m(x)/M(x) = V(x)/Vm = N/N A , (2)

where V(x) is the volume of substance X(l), V m is the molar volume of gas at n.o. (22.4 l / mol), N - number of particles, N A - Avogadro's constant (6.02 10 23).

Example 1. Determine the mass of sodium iodide NaI with a quantity of 0.6 mol.

Example 2. Determine the amount of atomic boron substance contained in sodium tetraborate Na 2 B 4 O 7 weighing 40.4 g.

m (Na 2 B 4 O 7) \u003d 40.4 g.

The molar mass of sodium tetraborate is 202 g/mol.

Determine the amount of substance Na 2 B 4 O 7:

n (Na 2 B 4 O 7) \u003d m (Na 2 B 4 O 7) / M (Na 2 B 4 O 7) \u003d 40.4 / 202 \u003d 0.2 mol.

Recall that 1 mol of sodium tetraborate molecule contains 2 mol of sodium atoms, 4 mol of boron atoms and 7 mol of oxygen atoms (see the formula of sodium tetraborate).

Then the amount of substance of atomic boron is equal to:

n (B) \u003d 4 n (Na 2 B 4 O 7) \u003d 4 0.2 \u003d 0.8 mol.

Instruction

The unit of molecular weight is 1/12 of the mass of an atom, which is conventionally taken as 12. The molecular weight is the total relative atomic mass of all the atoms in the molecule, and it is very easy to calculate.

And there is the easiest option if you know the substance. Take the periodic table, look at the molecular weight of each element included in. For example, for hydrogen it is 1, - 16. And to find the molecular weight of the whole substance (let's take for example water, which consists of two hydrogen molecules and one), simply add the masses of all the elements included in it. For water: M(H2O) = 2M(H)+M(O) = 2 1+16 = 18 a. eat.

Helpful advice

As you can see, finding the molecular weight can be very simple. The main thing is not to confuse it with the molar mass of a substance - they are numerically equal to each other, but have different units of measurement and physical meaning.

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Determine the molecular formula of a hydrocarbon if

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Experience as a teacher

In order to determine mass atom, find the molar mass of a monatomic substance using the periodic table. Then divide this mass by Avogadro's number (6.022 10^(23)). This will be the mass of the atom, in the units in which the molar mass was measured. The mass of an atom of a gas is found in terms of its volume, which is easy to measure.

You will need

To determine the mass of an atom of a substance, take the periodic table, tape measure or ruler, pressure gauge, thermometer.

Instruction

Determining the mass of an atom solid body or To determine the mass of an atom of a substance, determine it (what it consists of). In the periodic table, find the cell that describes the corresponding element. Find the mass of one mole of this substance in grams per mole that is in this cell (this number corresponds to the mass of the atom in atomic mass units). Divide the molar mass of the substance by 6.022 10^(23) (Avogadro's number), the result is the given substance in grams. The mass of an atom can also be determined in another way. To do this, multiply the atomic mass of a substance in atomic mass units taken in the periodic table by the number 1.66 10^(-24). Get the mass of one atom in grams.

Determining the mass of an atom of gas In the event that there is an unknown gas in the vessel, determine its mass in grams by weighing the empty vessel and the vessel with gas, and find the difference between their masses. After that, measure the volume of the vessel using a ruler or tape measure, followed by calculations or other methods. Express the result in . Use a manometer to measure the pressure of the gas inside the vessel, and measure its temperature with a thermometer. If the thermometer scale is calibrated in Celsius, determine the temperature value in Kelvin. To do this, add the number 273 to the temperature value on the thermometer scale.

Determining the molar mass of a substance from the mass of a molecule If you know the mass of one molecule in grams, multiply it by the Avogadro number 6.022 10^(23), which is equal to the number of molecules in one mole of the substance. The result will be the substance in grams per mole. Having found it in the periodic table, if necessary, determine the substance itself, if it is simple (consists of a monatomic molecule).

Determination of the molar mass of a gas Take a vessel of known volume and put some mass of gas into it. To do this, first pump the gas out of it and weigh it, and then pump the gas in and weigh it again. Then measure the gas pressure in pascals with a thermometer and its temperature. To convert Celsius to , add 273 to them. In order to find the molar mass, by converting the Clapeyron-Mendeleev equation, take the mass of the gas in grams, multiply it by the temperature and the number 8.31, which is universal. Divide the resulting number by the pressure in cubic meters (M = m 8.31 T / (P V)). The result will be the molar mass of the gas in grams per mole.

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molar masses of substances table

To find the molar mass substances, determine its chemical formula and, using the periodic table of Mendeleev, calculate its molecular mass. It is numerically equal to the molar mass substances in grams per mole. If the mass of one molecule is known substances, convert it to grams and multiply by 6.022 10^23 (Avogadro's number). molar mass gas can be found using the equation of state ideal gas.

You will need

periodic table, manometer, thermometer, scales.

Instruction

Determination of molar mass by chemical formula. Find the elements in the periodic table of Mendeleev that correspond to the atoms that the molecule consists of substances. If the molecule substances monoatomic, then this will be his. If not, find the atomic number of each element, and add up those masses. The result will be the molar mass substances, expressed in grams per mole.

Molar mass determination substances by the mass of one molecule. In the event that the mass of one molecule is known, convert it to, then multiply by the number of molecules in one mole of any substances, which is 6.022 10^23 (Avogadro's number). Get a molar mass substances in grams per mole.

Determination of the molar mass of a gas. Take a cylinder that can be hermetically sealed with a known volume, which is translated into. Use the pump to pump out the gas from it, and weigh the empty cylinder on the scales. Then fill it with the gas whose molar mass is being measured. Weigh the balloon again. The difference in the masses of the empty and filled with gas cylinder will be the mass of gas, express it in grams.
Using a pressure gauge, measure the gas pressure inside the cylinder, to do this, attach it to the gas injection hole. You can immediately use a cylinder with a built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in pascals.

Wait a while for the gas inside the cylinder to equalize with the temperature environment and measure it with a thermometer. Convert the indicator from degrees Celsius to kelvins, for which add the number 273 to the measured value.
Multiply the mass of the gas by the temperature and the universal gas constant (8.31). Divide the resulting number sequentially by the pressure and volume values (M \u003d m 8.31 T / (P V)). The result will be the molar mass of the gas in grams per mole.

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determination of molar mass

Molecular weight is the molecular weight, which can also be called the value of the mass of a molecule. The molecular weight is expressed in atomic mass units. If we disassemble the value of the molecular weight in parts, it turns out that the sum of the masses of all the atoms that make up the molecule is its molecular weight. mass. If we talk about units of mass, then mostly all measurements are made in grams.

Instruction

Molecular weight itself is related to the concept of a molecule. But it cannot be said that this condition can be applied only to those where the molecule, for example, hydrogen, is located separately. For cases where the molecules are not separate from the rest, but in close relationship, all of the above conditions and definitions are also valid.

To start, to define mass hydrogen, you will need - either, in which hydrogen is composed and from which it can be easily isolated. This can be some kind of alcohol solution or another mixture, some of the components of which, under certain conditions, change their state and easily free the solution from its presence. Find a solution from which you can vaporize the necessary or unnecessary substances by heating. This is the most easy way. Now decide whether you will evaporate a substance that you do not need or whether it will be hydrogen, molecular mass which you plan to measure. If an unnecessary substance evaporates, it's okay that it is not toxic. in the case of the evaporation of the desired substance, you need equipment so that all the evaporation is preserved in the flask.

After you have separated everything unnecessary from the composition, proceed to measurements. For this, Avogadro's number will suit you. It is with its help that you can calculate the relative atomic and molecular mass hydrogen. Find all the options you need hydrogen which are present in any table, determine the density of the resulting gas, as it will come in handy for one of the formulas. Then substitute all the results obtained and, if necessary, change the unit of measurement to , as already mentioned above.

The concept of molecular weight is most relevant when it comes to polymers. It is for them that it is more important to introduce the concept of average molecular weight, due to the heterogeneity of the molecules that make up their composition. Also, by the average molecular weight, one can judge how high the degree of polymerization of a particular substance is.

Related videos

Molecular mass is the mass of a molecule of a substance, expressed in atomic units. Often the problem arises: to determine the molecular weight. How can I do that?

Instruction

If you know, then the problem is solved elementarily. All you need is the Periodic Table. For example, you want to find the molecular weight of chloride. Write the formula of the substance: CaCl2. According to the periodic table, establish the atomic mass of each element included in its composition. For calcium, it is (rounded) 40, for (also rounded) - 35.5. Given index 2, find: 40 + 35.5 * 2 \u003d 111 a.m.u. (atomic mass units).

But what about in cases where the exact substance is unknown? Here you can act in different ways. One of the most effective (and at the same time, simple) is the so-called “osmotic pressure method”. It is based on osmosis, which consists in the fact that solvent molecules can penetrate a semi-impermeable, while solute molecules cannot penetrate through it. The value of osmotic pressure can be measured, and it is directly proportional to the concentration of the molecules of the substance under study (that is, their number per unit volume of the solution).

Some people are familiar with the universal Mendeleev-Clapeyron equation, which describes the state of the so-called "ideal gas". It looks like this: PVm = MRT. Van't Hoff's formula is very similar to it: P = CRT, where P is the osmotic pressure, C is molar concentration solute, R is the universal gas constant, T is the temperature in degrees Kelvin. This similarity is not accidental. It was as a result of the work of van't Hoff that it became clear that molecules (or ions) behave as if they were in a gas (with the same volume).

By measuring the value of the osmotic pressure, it is possible to simply calculate the molar concentration: С=P/RT. And then, knowing also the mass of the substance in the solution, find its molecular weight. Suppose it has been experimentally established that the molar concentration of the already mentioned substance is 0.2. At the same time, in a solution of 22.2 grams of this substance. What is its molecular weight? 22.2/0.2 = 111 amu - exactly the same as the previously mentioned calcium chloride.

Related videos

Molecular mass substances is the mass of a molecule, expressed in atomic units and numerically equal to the molar mass. In calculations in chemistry, physics and technology, the calculation of the values of the molar mass of various substances is often used.

You will need

- periodic table;
- table of molecular weights;
- table of cryoscopic constant values.

Instruction

Find the desired element in the periodic table. Pay attention to fractional numbers under its sign. For example, O has in the cell numerical value, equal to 15.9994. This is the atomic mass of the element. nuclear mass must be multiplied by the index of the element. The index shows how much of the element is contained in the substance.

If a complex is given, then multiply the atomic mass of each element by its index (if there is one atom of one or another element and there is no index, respectively, then multiply by one) and add the resulting atomic masses. For example, water is calculated as follows - MH2O = 2 MH + MO ≈ 2 1 + 16 = 18 a. eat.

Calculate the molar mass using suitable formulas and equate it to the molecular one. Change units from g/mol to a.m.u. Given pressure, volume, absolute Kelvin temperature, and mass, calculate the molar mass gas according to the Mendeleev-Claiperon equation M=(m∙R∙T)/(P∙V), in which M is the molecular () in amu, R is the universal gas constant.

Calculate molar mass according to the formula M=m/n, where m is the mass of any given substances, n - chemical quantity substances. Express Quantity substances through the Avogadro number n=N/NA or using the volume n=V/VM. Plug into the formula above.

Find the molecular mass gas, if only the value of its volume is given. To do this, take a sealed cylinder of known volume and pump out of it

Mole and atomic weight of a molecule

Set the chemical formula of the molecule

Find the atomic weight of each atom in the molecule. You can find this information in the Periodic Table; it is usually a decimal number above or below the element's symbol.
For example, the atomic mass of hydrogen is 1.0079 and the atomic mass of oxygen is 15.999. One mole of each element weighs the same amount in grams.
Further, based on the formula already known to us, we calculate the sum of all atoms of the molecule.
In particular, a water molecule contains two hydrogen atoms and one oxygen atom. In this case, the molar mass of water is 1.0079 + 1.0079 + 15.999, or 18.0148 g per mol (g/mol).
Divide the sum of the molecules by the mole or Avogadro's number (6.022 x 10^23).
For example, 18.0148 / 6.022 x 10^23 = 2.991 x 10^23. Thus, one water molecule weighs 2.991 x 10^23 g.

As can be seen from the above example, using the Periodic Table of Mendeleev, you can get all the necessary indicators to calculate the mass of molecules of various substances.