Law of Definite Proportions or Proust's Law

Law of definite proportions or Proust's law

The law of definite proportions, also known as the law of constant proportions or Proust's law, states that in a given chemical compound, the elements combine in specific and constant proportions by mass.

In other words, the composition of a chemical compound is fixed and defined, and the constituent elements are present in a ratio of simple integers. This law is a fundamental pillar of chemistry and was one of the first stoichiometric laws to be stated.

Furthermore, since elemental conversion does not occur in a chemical reaction, this also means that the mass ratio of the various constituent elements that make up the compound is always constant.

Examples of the Law of Definite Proportions

Here are three examples that illustrate this law:

Water Synthesis (formation of Water Vapor)

Law of definite proportions or Proust's lawReaction: 2H2(g) + O2(g) -> 2H2O(g)

In this reaction, two volumes of hydrogen gas react with one volume of oxygen gas to form two volumes of water vapor. This complies with the law, since the initial and final volumes are related in a ratio of simple integers (2:1:2).

Ammonia Decomposition

Reaction: 2NH3(g) -> 3H2(g) + N2(g)

In this reaction, two volumes of ammonia gas decompose to produce three volumes of hydrogen gas and one volume of nitrogen gas. Again, this reaction satisfies the law, since the initial and final volumes are related in a simple integer ratio (2:3:1).

Decomposition of Potassium Chlorate

Reaction: 2KClO3(s) -> 2KCl(s) + 3O2(g)

In this reaction, two moles of solid potassium chlorate decompose to produce two moles of solid potassium chloride and three moles of gaseous oxygen. The conversion of moles into volumes at standard conditions also complies with the aforementioned law.

Who Stated the Law of Definite Proportions?

Joseph Louis Proust published the law of definite proportions in 1797.

This law was implicitly recognized in quantitative analyzes by other prominent chemists in the 18th century, and was made explicit and confirmed by the French chemist Joseph Louis Proust.

Proust carried out numerous investigations analyzing the composition of a wide variety of chemical compounds. With them, he discovered that the mass of each of the chemical elements maintained the same proportion before and after forming the compound.

Proust's law contradicted the conclusions of Claude Louis Berthollet (1748-1822). Berthollet believed that the compositional relationships of the chemical elements in a compound depended on the conditions of formation. This discrepancy generated a strong controversy at the beginning of the 19th century.

Among those who supported Proust were Thomas Thomson, John Dalton, who based Dalton's atomic model on this law, and the Swedish chemist Jöns Jacob Berzelius.

In 1914, the Russian chemist Nikolai Semenovich Kurnakov concluded that Proust's stoichiometric compounds should be considered special types of phase classes, which he called daltonides after John Dalton.

However, intermetallic compounds and some metal oxides are also known to change the ratio of the component elements within a certain range, and are called non-stoichiometric compounds or Beltré compounds.

Importance of the Law of Definite Proportions

Proust's law is of utmost importance in chemistry. It provides a solid foundation for stoichiometry, which is the study of the quantitative relationships between reactants and products in a chemical reaction.

Based on this law, chemists can calculate the precise amounts of reactants needed to produce a given amount of product, and vice versa. This information is essential in chemical synthesis, drug formulation, materials manufacturing, and many other industrial and scientific applications.

This law is also fundamental for the identification and characterization of chemical compounds. When the mass ratio at which elements combine in a compound is known, chemical analyzes can be performed to determine the composition of an unknown substance.


Published: June 28, 2022
Last review: October 17, 2023