Proust’s Law

What is Proust’s law?

Proust’s law or law of definite proportions can be stated as follows:

When two elements combine to give rise to another compound, they always do so in a constant mass ratio.

In 1799, the French chemist Joseph Louis Proust (1754-1826) found in different samples of a compound the same elements in the same mass ratio.

Proust’s experiment

Proust studied the copper carbonate, the two tin oxides and the two iron sulfides to demonstrate this grade.

He did this by making artificial copper carbonate (CuCO3) and comparing it with natural copper carbonate. In doing so, he showed that each had the same weight ratio between the three elements involved.

Between the two types of the other compounds, Proust showed that there are no intermediate compounds between them.

The following table shows an example of the mass composition of magnesium oxides and their proportion.

Mg (g)O (g)Ratio (Mg/O)

From the experimental data in the table above, it follows that the combination ratio between the mass of the elements magnesium (Mg) and oxygen (O) is constant and equal to 1.5.

Proust published this work in 1794, which contradicted the conclusions of his famous opponent Claude Louis Berthollet. The latter argued that the proportions of combination of the elements in a compound depended on the conditions of its synthesis.

Proust’s law was not accepted until 1812, when the Swedish chemist Jöns Jacob Berzelius supported Proust’s proposal and obtained a broad consensus among the scientific community.

Compounds that satisfy the law of definite proportions are called ‘daltonides’ after John Dalton, who incorporated Proust’s law into his atomic theory by stating that compounds are formed by the union of atoms of different elements in a simple numerical ratio.

Explanation of Proust’s law

Although Proust was correct in his observations, the cause of why reagents behave in this way was not clear until the English chemist John Dalton formulated his atomic theory in 1803.

According to Dalton, a fixed number of atoms of one substance always combined with a fixed number of atoms of another substance to form a compound.

Dalton realized that substances should combine in the same proportions by weight as the proportions by weight of their atoms.

Other chemists had already observed that pure substances combine in fixed proportions. They called this finding the law of definite (or constant) proportions.

Therefore, Dalton’s theory satisfactorily explained Proust’s law.

Applications of Proust’s law

Proust’s law is applied to obtain the so-called centesimal composition of a compound, i.e. the weight percentage that each element represents within the molecule.

Limitations on Proust’s law

Certain solid compounds show a slight variation in their composition and therefore do not obey Proust’s law. These are called non-stoichiometric compounds or bertolites, named after Berthollet.

The cause of these variations is the crystallographic structure of the compounds.

Video about Proust’s Law