Stoichiometry

Stoichiometry, the doctrine of weight and volume relationships in which substances enter into chemical interaction. As early as 1749-56, M. V. Lomonosov emphasized the importance of studying the weight quantities of reacting substances. The law of conservation of matter, stated by Lomonosov and confirmed by the experiments of Lavoisier (see), forms the basis of modern stoichiometric concepts, as does the law of constant composition of chemical compounds, first clearly formulated by J. Proust (1754-1826). Extensive researches by Richter (1762-1807) on the quantities of acids and bases needed for mutual neutralization (see Alkalimetry) led to the concept of combining weight or equivalent for a given substance or element. By equivalent weight is meant that quantity of an element which can combine with one weight unit of hydrogen, the equivalent of which was taken as the unit. Subsequently, Berzelius (1779-1848), in determining the equivalents of many elements, found it more expedient to take oxygen as the unit of comparison, as an element that readily forms compounds with other elements, and its equivalent was taken as 8, which was later adopted by the international committee on atomic weights. The law of multiple proportions, discovered by J. Dalton (1766-1844), stating that if one substance forms several compounds with another, the weight quantities of the first substance in these compounds are in simple multiple ratios to each other, served as a further step in the development of stoichiometric concepts. The basic stoichiometric regularities for gaseous substances were discovered by Gay-Lussac (1778-1851). All of them can be reduced to the fact that the volumes of gases combining with each other are in simple multiple ratios to each other and to the volume of the gaseous product formed in the reaction. Finally, Avogadro's law (see) provided the guiding thread for further refinement and determination of molecular and atomic weights of elements, to which C. Gerhardt and A. Laurent (1840) contributed considerably by beginning systematic determinations of vapor densities of elements for determining their molecular weights. In 1858, S. Cannizzaro gave the definition of an atom as the smallest quantity of an element contained in the particles of the greatest possible number of compounds of that element. The discovery of the regularity between the depression of the freezing point of a solution and the amount of dissolved substance (Raoult, 1882) gave researchers an exact method (see Cryoscopy) for determining the molecular weights of solids, which contributed to the final establishment of the concept of molecular weight as the weight of the smallest quantity of substance or element capable of actual existence. At the present time, in the formula of a substance we have, as it were, a brief record not only of its qualitative but also of its quantitative composition. Knowing the equation of a particular process, i.e., the formulas of the initial substances and the formulas of the substances formed, as well as the stoichiometric coefficients showing the number of molecules entering and formed as a result of the reaction, we can calculate the quantities of substances obtained if we know the quantities taken, which is of great importance in the practice of chemical production and laboratory research.