Concentration- value characterizing the quantitative composition of the solution.

The concentration of a solute (not a solution) is the ratio of the amount of a solute or its mass to the volume of a solution (mol / l, g / l), that is, this is the ratio of heterogeneous quantities.

Those quantities that are the ratio of the same type of quantities (the ratio of the mass of a solute to the mass of a solution, the ratio of the volume of a solute to the volume of a solution) are correctly called shares. However on practice for both types of expression of composition, the term is used concentration and talk about the concentration of solutions.

There are many ways to express the concentration of solutions.

Mass fraction (also called percentage concentration)

Mass fraction - the ratio of the mass of the solute to the mass of the solution. The mass fraction is measured in fractions of a unit.

m 1 - mass of the dissolved substance, g (kg);

m is the total mass of the solution, g (kg).

Mass fraction of the solute w (B) is usually expressed as a fraction of a unit or as a percentage. For example, the mass fraction of the dissolved substance - CaCl 2 in water is 0.06 or 6%. This means that a solution of calcium chloride weighing 100 g contains calcium chloride weighing 6 g and water weighing 94 g.

Example: How many grams of sodium sulfate and water are needed to prepare 300 g of a 5% solution?

Solution: m (Na 2 SO 4) \u003d w (Na 2 SO 4) / 100 \u003d (5 300) / 100 \u003d 15 (g)

where w (Na 2 SO 4)) is the mass fraction in%, m is the mass of the solution in g m (H 2 O) \u003d 300 g - 15 g \u003d 285 g.

Thus, to prepare 300 g of a 5% sodium sulfate solution, you need to take 15 g of Na 2 SO 4) and 285 g of water.

Mass percentage of the component, ω%

ω % =(m i /Σm i)*100

Volume fraction

Volume fraction - the ratio of the volume of a solute to the volume of a solution. The volume fraction is measured in fractions of a unit or as a percentage.

V 1 - the volume of the dissolved substance, l;

V is the total volume of the solution, l.

There are hydrometers designed to determine the concentration of solutions of certain substances. Such hydrometers are graduated not in density values, but directly in the values ​​of the concentration of the solution. For common solutions ethyl alcohol, the concentration of which is usually expressed as a percentage by volume, such hydrometers are called alcohol meters.

Molarity (molar volume concentration)

Molar concentration - the amount of solute (number of moles) per unit volume of the solution. Molar concentration in is measured in mol/l (M) or mmol/l (mM). The expression in "molarity" is also common. So, a solution with a concentration of 0.5 mol / l is called 0.5 molar.

ν - amount of dissolved substance, mol;

V is the total volume of the solution, l.

Molar concentration is measured in mol/l and is denoted by "M". For example, 2 M NaOH is a 2 molar solution of sodium hydroxide. One liter of such a solution contains 2 mol of a substance or 80 g.

Example: What mass of potassium chromate K 2 CrO 4 need to take to prepare 1.2 liters of 0.1 M solution?

Solution: M (K 2 CrO 4) \u003d C (K 2 CrO 4) V M (K 2 CrO 4) \u003d 0.1 mol / l 1.2 l 194 g / mol "23.3 g.

Thus, to prepare 1.2 liters of a 0.1 M solution, you need to take 23.3 g of K 2 CrO 4 and dissolve in water, and bring the volume to 1.2 liters.

The article deals with such a concept as mass fraction. Methods for its calculation are given. Definitions of quantities similar in sound, but different in physical meaning, are also described. These are mass fractions for element and output.

Cradle of life - solution

Water is the source of life on our beautiful blue planet. This expression can be found quite often. However, few people, except for specialists, think: in fact, a solution of substances, and not chemically pure water, became the substrate for the development of the first biological systems. Surely, in popular literature or a program, the reader has come across the expression "primary broth."

The sources that gave impetus to the development of life in the form of complex organic molecules are still being debated. Some even suggest not just a natural and very fortunate coincidence, but cosmic intervention. Moreover, we are not talking about mythical aliens at all, but about specific conditions for the creation of these molecules, which can exist only on the surface of small cosmic bodies devoid of an atmosphere - comets and asteroids. Thus, it would be more correct to say that the solution of organic molecules is the cradle of all life.

Water as a chemically pure substance

Despite the huge salty oceans and seas, fresh lakes and rivers, water in its chemically pure form is extremely rare, mainly in special laboratories. Recall that in the domestic scientific tradition, a chemically pure substance is a substance that contains no more than ten to the minus sixth power of the mass fraction of impurities.

Obtaining a mass that is absolutely free from foreign components requires incredible costs and rarely justifies itself. It is used only in individual industries, where even one foreign atom can spoil the experiment. Note that semiconductor elements, which form the basis of today's miniature technology (including smartphones and tablets), are very sensitive to impurities. In their creation, completely uncontaminated solvents are needed. However, compared to the entire liquid of the planet, this is negligible. How is it that the common water that permeates our planet is so rare in its pure form? Let's explain below.

Ideal Solvent

The answer to the question posed in the previous section is incredibly simple. Water has polar molecules. This means that in every smallest particle of this liquid, the positive and negative poles are not much, but separated. At the same time, structures that arise even in liquid water create additional (so-called hydrogen) bonds. And in total it gives the following result. The substance that enters the water (no matter what charge it has) is pulled apart by the molecules of the liquid. Each particle of the dissolved impurity is enveloped by either negative or positive sides of water molecules. Thus, this unique liquid is capable of dissolving a very large number of a wide variety of substances.

The concept of mass fraction in solution

The resulting solution contains some of the impurities, called "mass fraction". Although this expression is not often found. Another term commonly used is "concentration". The mass fraction is determined by a specific ratio. We will not give a formulaic expression, it is quite simple, we will explain better the physical meaning. This is the ratio of two masses - impurities to the solution. Mass fraction is a dimensionless quantity. It is expressed in different ways depending on the specific tasks. That is, in fractions of a unit, if the formula contains only the ratio of masses, and in percentage - if the result is multiplied by 100%.

Solubility

In addition to H 2 O, other solvents are also used. In addition, there are substances that fundamentally do not give up their molecules to water. But they easily dissolve in gasoline or hot sulfuric acid.

There are special tables that show how much of a particular material will remain in the liquid. This indicator is called solubility, and it depends on temperature. The higher it is, the more actively the atoms or molecules of the solvent move, and the more impurities it is able to absorb.

Options for determining the proportion of a solute in a solution

Since the tasks of chemists and technologists, as well as engineers and physicists, can be different, the part of the dissolved substance in water is defined in different ways. The volume fraction is calculated as the volume of the impurity to the total volume of the solution. A different parameter is used, but the principle remains the same.

The volume fraction retains dimensionlessness, being expressed either in fractions of a unit or as a percentage. Molarity (also called "molar volume concentration") is the number of moles of a solute in a given volume of solution. This definition already involves two different parameters of one system, and the dimension of this quantity is different. It is expressed in moles per litre. Just in case, we recall that a mole is the amount of a substance containing about ten to the twenty-third degree of molecules or atoms.

The concept of the mass fraction of an element

This value is only indirectly related to solutions. The mass fraction of an element differs from the concept discussed above. Any complex chemical compound consists of two or more elements. Each has its own relative weight. This value can be found in Mendeleev's chemical system. There it is indicated in non-integer numbers, but for approximate tasks the value can be rounded off. The composition of a complex substance includes a certain number of atoms of each type. For example, in water (H 2 O) there are two hydrogen atoms and one oxygen. The ratio between the relative mass of the whole substance and the given element in percent will be the mass fraction of the element.

To the inexperienced reader, these two concepts may seem close. And quite often they are confused with each other. The mass fraction of the yield does not refer to solutions, but to reactions. Any chemical process always proceeds with the receipt of specific products. Their yield is calculated by formulas depending on the reactants and process conditions. Unlike simply mass fraction, this value is not so easy to determine. Theoretical calculations suggest the maximum possible amount of the substance of the reaction product. However, practice always yields a slightly lower value. The reasons for this discrepancy lie in the distribution of energies among even highly heated molecules.

Thus, there will always be the "coldest" particles that cannot enter into a reaction and remain in their original state. The physical meaning of the mass fraction of the yield is the percentage of the actually obtained substance from the theoretically calculated one. The formula is incredibly simple. The mass of the practically obtained product is divided by the mass of the practically calculated one, the whole expression is multiplied by one hundred percent. The mass fraction of the yield is determined by the number of moles of the reactant. Don't forget about it. The fact is that one mole of a substance is a certain number of its atoms or molecules. According to the law of conservation of matter, twenty molecules of water cannot make thirty molecules of sulfuric acid, so the problems are calculated in this way. From the number of moles of the initial component, the mass is derived, which is theoretically possible for the result. Then, knowing how much of the reaction product was actually obtained, the mass fraction of the yield is determined using the formula described above.

Mass fraction- the ratio of the mass of the solute to the mass of the solution. The mass fraction is measured in fractions of a unit.

m 1 - mass of the dissolved substance, g;

m is the total mass of the solution, g.

Mass percentage of the component, m%

m % =(m i /Σm i)*100

In binary solutions, there is often an unambiguous (functional) relationship between the density of the solution and its concentration (at a given temperature). This makes it possible to determine in practice the concentration of important solutions using a densimeter (alcohol meter, saccharimeter, lactometer). Some hydrometers are not graduated in density values, but directly in the concentration of the solution (alcohol, fat in milk, sugar). It should be borne in mind that for some substances the density curve of the solution has a maximum, in this case 2 measurements are carried out: direct, and with a slight dilution of the solution.

Often, to express the concentration (for example, sulfuric acid in the electrolyte of batteries), they simply use their density. Hydrometers (densimeters, density meters) are common, designed to determine the concentration of solutions of substances.

Volume fraction

Volume fraction is the ratio of the volume of the solute to the volume of the solution. The volume fraction is measured in fractions of a unit or as a percentage.

V 1 - the volume of the dissolved substance, l;

V is the total volume of the solution, l.

As mentioned above, there are hydrometers designed to determine the concentration of solutions of certain substances. Such hydrometers are not graduated in terms of density, but directly in the concentration of the solution. For common solutions of ethyl alcohol, the concentration of which is usually expressed as a percentage by volume, such hydrometers are called alcohol meters or andrometers.

Molarity (molar volume concentration)

Molar concentration - the amount of solute (number of moles) per unit volume of the solution. Molar concentration in the SI system is measured in mol / m³, but in practice it is much more often expressed in mol / l or mmol / l. The expression in "molarity" is also common. Possible other designation of molar concentration C M, which is usually denoted M. So, a solution with a concentration of 0.5 mol / l is called 0.5-molar. Note: the unit "mol" is not declined by cases. After the number, they write "mol", just as after the number they write "cm", "kg", etc.

V is the total volume of the solution, l.

Normal concentration (molar equivalent concentration)

Normal concentration- the number of equivalents of a given substance in 1 liter of solution. The normal concentration is expressed in mol-eq / l or g-eq / l (meaning mole equivalents). To record the concentration of such solutions, the abbreviations " n" or " N". For example, a solution containing 0.1 mol-eq / l is called decinormal and is written as 0.1 n.

ν - amount of dissolved substance, mol;

V is the total volume of the solution, l;

z is the equivalence number.

The normal concentration may differ depending on the reaction in which the substance is involved. For example, a one molar solution of H 2 SO 4 will be one normal if it is intended to react with an alkali to form KHSO 4 hydrosulfate, and two normal if it is to react to form K 2 SO 4 .

The composition of air includes several different gases: oxygen, nitrogen, carbon dioxide, noble gases, water vapor and some other substances. The content of each of these gases in clean air is strictly defined.

In order to express the composition of a mixture of gases in numbers, i.e. quantitatively, a special value is used, which is called the volume fraction of gases in the mixture.

The volume fraction of gas in the mixture is denoted by the Greek letter - "phi".

The volume fraction of a gas in a mixture is the ratio of the volume of a given gas to the total volume of the mixture:

What does the volume fraction of gas in the mixture show, or, as they say, what is the physical meaning of this quantity? The volume fraction of a gas shows what part of the total volume of the mixture is occupied by a given gas.

If we were able to separate 100 liters of air into separate gaseous components, we would get about 78 liters of nitrogen, 21 liters of oxygen, 30 ml of carbon dioxide, the remaining volume would contain the so-called noble gases (mainly argon) and some others (Fig. 62).

Let's calculate the volume fractions of these gases in the air:

It is easy to see that the sum of the volume fractions of all gases in the mixture is always equal to 1, or 100%:

(nitrogen) + (acid) + (carbon gas) + (other gases) = 78% + 21% + 0.03% + 0.97% = 100%.

The air that we exhale is much poorer in oxygen (its volume fraction decreases to 16%), but the carbon dioxide content increases to 4%. This air is no longer suitable for breathing. That is why a room in which there are many people must be regularly ventilated.

In chemistry in production, one often has to deal with the inverse problem: to determine the volume of gas in a mixture from a known volume fraction.

Example. Calculate the volume of oxygen contained in 500 liters of air.

From the definition of the volume fraction of gas in the mixture, we express the volume of oxygen:

V(sour) = V(air) (acid).

Substitute the numbers in the equation and calculate the volume of oxygen:

V(acid) \u003d 500 (l) 0.21 \u003d 105 l.

By the way, for approximate calculations, the volume fraction of oxygen in the air can be taken equal to 0.2, or 20%.

When calculating the volume fraction of gases in a mixture, you can use a little trick. Knowing that the sum of the volume fractions is 100%, for the "last" gas in the mixture, this value can be calculated differently.

Task.An analysis of the atmosphere of Venus showed that 50 ml of Venusian "air" contains 48.5 ml of carbon dioxide and 1.5 ml of nitrogen. Calculate the volume fractions of gases in the planet's atmosphere.

Given:

V(mixture) = 50 ml,

V(carbon gas) = ​​48.5 ml,

V(nitrogen) = 1.5 ml.

To find:

(coal gas),

Decision

Calculate the volume fraction of carbon dioxide in the mixture. A-priory:

Let us calculate the volume fraction of nitrogen in the mixture, knowing that the sum of the volume fractions of gases in the mixture is 100%:

(carbon gas) + (nitrogen) = 100%,

(nitrogen) = 100% - (carbon gas) = ​​100% - 97% = 3%.

Answer.(carbon gas) = ​​97%, (nitrogen) = 3%.

What quantity is used to measure the content of components in mixtures of another type, for example, in solutions? It is clear that in this case it is inconvenient to use the volume fraction. A new value comes to the rescue, which you will learn about in the next lesson.

1. What is the volume fraction of a component in a gas mixture?

2. The volume fraction of argon in air is 0.9%. What volume of air is needed to produce 5 liters of argon?

3. When air was separated, 224 liters of nitrogen were obtained. What volumes of oxygen and carbon dioxide were obtained in this case?

4. The volume fraction of methane in natural gas is 92%. What volume of this gas mixture will contain 4.6 ml of methane?

5. Mixed 6 liters of oxygen and 2 liters of carbon dioxide. Find the volume fraction of each gas in the resulting mixture.