According to the expression of the law of constancy of composition,any chemically pure compound remains in the same quantitative composition, in no way dependent on the method of its production, which was first reported to the world by the scientist J. Proust in 1801-1808. The law appeared as a result of a dispute between the French chemists J. Proust and C. Berthollet. The first of them believed that the relationship between the elements of the resulting compound are of a constant nature, the second saw the variable nature of the compounds. A hundred years later, around 1912-1913, N.S. Kurnakov established the existence of compounds with a composition of variable nature, which he called "berthollids." To this group are listed crystalline compounds: phosphides, oxides, carbides and others. Compounds of a composition with a constant character at the suggestion of the scientist N.S. Kurnakov became known as "daltonids". The law is always valid for gaseous and liquid substances.
From the formulated law of constancy of compositionit follows logically that the elements of substances are connected with each other in strictly limited quantitative ratios. In connection with this, in chemistry there is the concept of an equivalent, which in Latin means "equivalent". In a word, the equivalent is the conditional particles of matter, which are a certain number of times smaller than the corresponding formula units. Any equivalent number corresponds to the nature of the reacting substances, the degree and type of chemical reaction. That is why distinguish the equivalent numbers of a particular element in the composition of a compound - for known groups, for ions or even molecules. In reactions of the exchange type, for example, the molar mass of the equivalent of a substance is determined by the stoichiometry of the reaction.
Usually, many elements are able to formseveral connections between each other. Therefore, the equivalent of the element, as well as the molar mass of the equivalent, can have different values, looking back at the fact that they were identified from the composition of the test compound. However, in such cases, different equivalents of the same element can be related to each other as relatively small integers. For example, the molar mass of the carbon equivalent found in the dioxide and carbon monoxide is different and is about 3 grams / mole and 6 grams / mole, and the ratio of the values found is 1: 2. As a rule, most compounds contain a molar mass of equivalents of hydrogen equal to one, and oxygen - eight grams per mole. Equivalent is the amount of matter where one mole of valence electrons is enclosed.
There are several methods that allow one to determine experimentally how large the molar mass is of the equivalent of any element:
- Direct method. It is based on the obtained data as a result of the synthesis of hydrogen and oxygen compounds of the element sought.
- Indirect method. Instead of hydrogen and oxygen, he uses other elements with a previously known equivalent.
- The method of displacement. It involves the removal of hydrogen from the acid solution with the help of a metal of a certain sample.
- Analytical method. It is based on the calculation of the mass fraction of a substance in one of its compounds.
- The electrochemical method uses electrolysis data.
The molar mass of the equivalent is used forthe implementation of quantitative calculations during chemical interactions between known substances. An important advantage here is that to solve the problem there is no need to use the equation of the chemical reaction, which, moreover, is difficult to write. It is only necessary to know that the participating chemicals interact, or the substance is a product of a chemical reaction.
