Solutions: concentration, mass fraction. Definition, calculation and recommendations

The mass concentration of the solution is one of the most common concepts in modern chemistry. In the article we will reveal the features of solutions, their types, application. Let's note some examples of calculation of different types of concentrations.

Features of solutions

A solution is a homogeneous system that has a variable composition. Of the two components of the solution, one always acts as a medium. It is in it that structural fragments of other substances will dissolve. It is called a solvent, within which molecules of the dissolved substance are located.

If two gaseous substances are mixed, then in this case no solvent is released. For each specific situation, special calculations are always made.

Obtaining homogeneous systems

To obtain homogeneous solutions, it is necessary to break up the dissolved substances into structural units. Only in this case will the systems be true. When crushed to small droplets, grains of sand that will be distributed in the medium, colloidal solutions, emulsions, suspensions are obtained.

Application of solutions

By the way, in construction, a mixture of sand, cement, water is also called a solution, but from the chemical point of view it is a suspension. The practical importance of solutions can be explained for various reasons.

Chemical reactions in liquid solutions occur in the volume of the solvent. This makes them available for reaction without any additional action on the system. In a mixture containing solid particles, it is impossible to carry out the reaction in full. To speed up the process, we will need to touch the particles at some points. To increase the reaction speed, the crystals are grinded in a mortar, then they are pressed. But it is not immediately possible to achieve the completeness of the process.

In the solution, the process proceeds differently. Molecules move freely, when they collide, chemical transformations take place. Energy, which begins to be released in this interaction, is accumulated by the solvent, the system practically does not warm up.

Physical properties and concentration of solutions

The mass fraction of the substance makes it possible to determine the quantitative ratio of the dissolved substance and the solvent taken for their preparation. Metal alloys, by the way, are also solutions, but solid, characterized by certain physical parameters.

Solutions have the ability to change the strength of the dissolved component. This makes them in demand in agriculture, medicine. For example, a solution of manganese (potassium permanganate) is used to treat abrasions and wounds in medium concentration. But its practical significance is also its insignificant concentration. Thus, the mass fraction of the substance 2-3% gives the solution a slight pink color, in demand for washing the stomach.

Dark purple crystals of potassium permanganate are not used for medical purposes, since they have strong oxidizing properties. In general, the intensity of the color is directly related to its concentration. The mass fraction of the substance makes it possible to regulate the toxicity of the prepared solution.

Mass fraction

How is this concentration calculated? The mass fraction of a substance is characterized by the ratio of the mass of the substance to the mass of the solution taken as a percentage. Their organoleptic properties are affected not only by what will be dissolved, but also by the quantitative index. For example, for a weak solution of common salt, there is almost no taste, and at high concentrations it manifests itself in varying degrees.

How is concentration determined in practice? The mass fraction of a substance in solution is considered in the school course of inorganic chemistry. Tasks for its definition are included in the test tasks for graduates of grade 9.

Here is an example of a task in which concentration is used.

Mass fraction of table salt 25%. The weight of the solution is 250 grams. Determine the mass of water contained in it. To perform the calculations, you first need to find out the mass of the substance. Based on the proportion, we find that the substances in the solution are 62.5 grams. To determine the mass of water, you subtract from 250 grams the mass of the substance itself, resulting in 187.5 g.

Types of concentrations

What is concentration? Mass fractions in solution can contain no more than one hundred percent. In chemistry, the term "concentration" implies a certain content of a dissolved substance. There are several of its variants: molar, mass concentration.

For example, if you need to prepare a solution of 80 grams of water and 20 grams of table salt and determine the mass fraction of the substance in the solution, you first need to determine the mass of the solution. It will be one hundred grams. The percentage of the substance is 20 percent.

We analyzed what is the mass fraction. The molar concentration assumes the ratio of the amount of substance to the volume of the solution taken. To prepare a solution with a given molar concentration, the mass of the substance is first determined. Then weigh the required amount and dissolve in a liter of solvent.

Calculation of the molar concentration

So, to prepare 2 liters of a solution with a concentration of 0.15 mol / l, first calculate the mass of salt that is contained in the solution. To do this, separate 0.15 mole by 2 liters, we obtain 0.075 moles. Now calculate the mass: 0.075 mol is multiplied by 58.5 g / mol. The result is 4.39 g.

Problems of Analytical Chemistry

As an applied chemical problem, analysis is considered. With its help, the composition of the mixture is revealed, diagnostic tests are carried out, rocks are analyzed. For this, it is necessary to determine the qualitative and quantitative composition of the solution.

Among those problems that are most often encountered in inorganic chemistry, we single out the definition of the concentration of one substance with a given value for another substance. With the help of experiments, it is possible to gradually add the sought solution to the solution in which the molar concentration is known. This process is called titration.

Solubility and solvents

The most common solvent is water. It completely dissolves bases, acids, salts, some organic compounds. It is water solutions that are the most common systems in nature. Water serves as a biological solvent. It is considered the basis for the flow of many media: blood, cytosols, intercellular fluids. Many types of animals and plants live precisely in the aquatic environment.

Solubility is the property of the substance to be dissolved in the chosen solvent. This is a complex phenomenon, which requires taking into account certain nuances and peculiarities of the structure of the solvent.

As good organic substances can be noted alcohols. They include hydroxy groups in their composition, and therefore have a high solubility.

Conclusion

Any liquid can be considered as a solvent. That is why they often talk about the mutual solubility of different liquid substances. For example, among organic substances we can mention the solubility in water of esters.

Different types of concentrations used in inorganic and organic chemistry, help to conduct qualitative and quantitative determinations of substances. The theory of solutions is in demand in analytical chemistry, pharmaceutics and modern medicine.