Law of definite proportions

Law of defined proportions . Enunciated by Louis Joseph Proust , based on experiments carried out in the early nineteenth century by what is also known as Proust’s Law. It states that when two or more elements combine to form a certain compound, they do so in an invariable weight ratio.

Explanation of the Law

This law can serve as an example of the application of the scientific method. In contact with air, iron corrodes and that other metals, such as zinc and aluminum , turn into different substances.

As an example, to obtain iron sulfate, we must combine iron and sulfur in the following proportion: 7 parts of iron, for 4 parts of sulfur. Thus we obtain 11 parts of iron sulfate .

According to the law we have:

7g iron + 4g sulfur = 11g iron sulfate

Combining 9gr. iron with 4 gr. sulfur, still we get 11 gr. iron sulfate, but 2 gr. of iron.

In the same way, when combining 7 gr. iron with 5 gr. sulfur, we will also get 11 gr. of iron sulfate, but now we will have 1 g of sulfur.

In this combination, the amount of iron and sulfur can be different from 7 gr. and 4 gr., respectively, but both substances always react in the ratio of 7 to 4.

This relationship can also be obtained by the atomic mass of the elements. Since the atomic mass of iron is 56 and that of sulfur 32. We have the proportion 56:32

For simplicity, each of these numbers is divided by the greatest common divisor and we arrive at the following result: 7: 4

Thus we can conclude that in the formation of this compound, the elements with the highest atomic mass participate in a greater proportion.

Through analysis of innumerable substances acquired by different processes it was possible to verify that the same substance always has the same qualitative and quantitative composition. For example, any water sample always has 88.9% oxygen and 11.1% by mass of hydrogen, combined in the same proportion.

Proust’s law was studied and approved and later extended to any chemical reaction .

Lavoisier and Proust’s laws consist of measuring the amount of a substance in the laboratory and industry, it is the guarantee that in a chemical process neither creation nor destruction of matter occurs, that is why it is called the law of conservation of mass .

Proust’s law is the guarantee of proportionality between the mass of the reactive substances and the products in a chemical reaction; for this reason it is called the law of Defined Proportions.

These laws, in industry and in the laboratory, serve both to calculate the amount of reagents in the preparation of substances and the amount of products that must be obtained.


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