The concentration of solutions - a concept that is quite common both in scientific circulation and in household vocabulary - characterizes the quantitative structure of the composition of a solution. In accordance with the standards and definitions of IUPAC (international registry of chemical compounds), concentration is understood as the ratio of the mass of a substance and a solution. This ratio is measured in units of mol / L, or g / L.
If we consider substances whose proportion can be expressed by the same values (for example, mass to mass), then their ratio is usually reflected in fractions, but in practice the use of the concept of concentration of solutions has become more widespread.
There are several options by which you can reflect the concentration. For solutions, the expression of the structure of the solution in mass and volume (for liquids) percentages is most often used, the concentration in moles can be expressed, in addition, the concentration of solutions can be expressed in gram equivalents. In some cases, the option of reflecting the concentration using a titer or an indicator of molarity is used.
As a rule, the concentration of solutions that do not require great accuracy in their expression is shown by mass percent. Where high accuracy is needed, moths, captions, or gram equivalents are used.
When using mass percent, the rule should be applied according to which the expression of concentration is indicated in the same units, for example, how many grams of a substance are contained in a certain number of grams of a solution. It is impossible to combine reflection in grams and volume units (for example: the number of grams of a substance in 100 milliliters of a solution).
For example, if we need to express the composition of a 10% solution of sodium chloride, the formula of which is NaCl, then it should be understood that a 100 gram solution includes 10 g of salt and 90 g of water. In the event that the concentration is specified in units of the percentage of the substance (for example, a 25% solution), and it is necessary to use such an amount of the solution that it contains some exactly specified amount of the substance (for example, 5 g), then the solution mass (in this example, it is 20 g).
For each version of the reflection of concentration there is its own formula. So, if the percentage concentration of the solution is used, the formula has the following form:
C = m1 / m, in which: m1 is the mass of a substance dissolved in a given solution, and expressed in grams or kilograms, m is the mass of the solution, expressed in the same units.
The molar concentration of the substance in the solution is determined by the formula: C (M) = n / V, where n is the amount of substance in moles, V is the volume of solution in liters. When using gram equivalents, the formula takes the form: C (N) = z / V, in which V is the volume of the solution, expressed in liters, z is the number of equivalence.
In cases where the density of the solution is known, it is more convenient to reflect it in volume. But this is only in the case when it comes to sufficiently concentrated solutions (less than 1%), since the error in this case can be insignificant.
The concentration expressed in the number of moles in chemistry is called molarity. If they say: “unipolar solution”, then this means that 1 mole of substance is contained in 1 liter of liquid. Such a solution is also called simply “molar”.
If the concentration is expressed by the number of gram equivalents that are contained in a unit volume of liquid, then in this case the expression “normality” is used, which shows that, for example, 1 gram equivalent is in 1 liter of liquid. Most often, such a solution is simply called normal.
The article considers only some basic methods of expressing concentrations. In other cases, when it comes to complex solutions, other units can be used for this.