Mass concentration of a solution is one of the common concepts in modern chemistry. In the article we will identify the features of solutions, their types, application. Note some examples of calculating different types of concentrations.
Features of solutions
A solution is a homogeneous system with a variable composition. Of the two components of the solution, one always acts as a medium. It is in it that the structural fragments of other substances will dissolve. It is called a solvent, inside of which the molecules of the dissolved substance are located.
If two gaseous substances are mixed, then in this case no solvent is emitted. For each specific situation, special calculations are always carried out.
Getting homogeneous systems
To obtain homogeneous solutions, crushing of dissolved substances into structural units is necessary. Only then will the systems be true. When crushed to small droplets, grains of sand, which will be distributed in the medium, get colloidal solutions, emulsions, suspensions.
The use of solutions
By the way, in construction, a mixture of sand, cement, water is also called a solution, but from a chemical point of view it is a suspension. The practical significance of the 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, it will require the contact of the particles at some points. To increase the reaction rate, the crystals are ground in a mortar, then they are pressed. But it is not immediately possible to achieve the completeness of the process.
In solution, the process proceeds differently. Molecules move freely, and chemical collisions occur during their collisions. The energy that begins to be released in this interaction is accumulated by the solvent, the system practically does not heat up.
Physical properties and concentration of solutions
The mass fraction of the substance allows you 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 potassium permanganate (potassium permanganate) is used to treat abrasions and wounds in an average concentration. But its insignificant concentration is also of practical importance. So, the mass fraction of a substance of 2-3% gives the solution a slightly pink color, which is in demand for gastric lavage.
Dark violet crystals of potassium permanganate are not used for medical purposes, since they have strong oxidizing properties. In general, the color intensity is directly related to what its concentration is. The mass fraction of the substance allows you to adjust the toxicity of the finished 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 influenced not only by what will dissolve, but also by a quantitative indicator. For example, an aftertaste is almost not characteristic of a weak solution of table salt, and at high concentrations it appears to varying degrees.
How is concentration determined in practice? The mass fraction of the substance in the solution is considered in the school course of inorganic chemistry. Tasks for its definition are included in test assignments for graduates of grade 9.
Let us give an example of a task in which concentration is used.
Mass fraction of table salt 25%. The mass of the solution is 250 grams. Determine the mass of water contained in it. To carry out 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 need to subtract the mass of the substance from 250 grams, as a result we get 187.5 g.
Types of concentrations
What is concentration? Mass fractions in the solution may contain no more than one hundred percent. In chemistry, the term "concentration" implies a certain content of solute. There are several options: 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. She will be one hundred grams. The percentage of the substance is 20 percent.
We have analyzed what constitutes a mass fraction. Molar concentration implies the ratio of the amount of substance to the volume of the taken solution. To prepare a solution with a given molar concentration, the mass of the substance is first determined. Then weigh the right amount and dissolve in a liter of solvent.
Calculation of molar concentration
So, to prepare 2 liters of a solution with a concentration of 0.15 mol / l, the mass of salt that is contained in the solution is first calculated. To do this, divide 0.15 mol into 2 liters, we get 0.075 mol. Now calculate the mass: 0.075 mol, multiply by 58.5 g / mol. The result is 4.39 g.
Tasks 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. To do this, you need to determine the qualitative and quantitative composition of the solution.
Among the tasks that are most often encountered in inorganic chemistry, we single out the determination of the concentration of one substance by a given value for another substance. With the help of the experiments, it is possible to carry out the gradual addition to the solution, in which the molar concentration is known, of the desired solution. This process is called titration.
Solubility and solvents
The most common solvent is water. It perfectly dissolves bases, acids, salts, and some organic compounds. It is aqueous solutions that are the most common systems in nature. Water acts as a biological solvent. It is considered the basis for the flow of many environments: blood, cytosol, intercellular fluids. Many types of animals and plants live in the aquatic environment.
Solubility is the property of a substance to dissolve in a selected solvent. This is a complex phenomenon that requires taking into account certain nuances and structural features of the solvent.
Alcohols can be mentioned as good organic substances. They include hydroxyl groups in their composition, therefore, they have high solubility.
Conclusion
Any liquid can be considered as a solvent. That is why they often talk about the mutual solubility of various liquid substances. For example, among organic substances, water solubility of esters can be mentioned.
Different types of concentrations used in inorganic and organic chemistry help to conduct qualitative and quantitative determination of substances. The theory of solutions is in demand in analytical chemistry, pharmaceuticals and modern medicine.