Proust's law

Proust's law, knowing this law is important for chemistry and in everyday life because it explains how the combination of the elements in a chemical reaction happens. Which follows a ratio of fixed proportions with respect to the mass of a compound. Understanding this law is simple you just have to make comparisons with aspects of reality. I invite you to read the article so that you can apply it.

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What is Proust's law?

It is known in classrooms as the law of definite proportions and the law of constant proportions. It is the basis for the application of stoichiometry. It expresses that the masses of the compounds after they react remain constant. That the constituents retain a fixed proportion by weight.

It says that there is a proportionality between the product of a reaction and the relative mass of the reactants. It asserts that an element is made up of the same type of atoms, with the same atomic weight. When several chemical elements are joined to form a compound, it has a definite and fixed proportion. This law combines in perfect union with the law of atomic theory.

Its application is useful and necessary in the measurement of the amount of substance in the laboratory, in industry. It is a guarantee that in a chemical process the creation of matter does not occur, only transformation. It is called a quantitative study. It has no dependence on the procedure used for the repair of the compound.

Proust law
Proust law

What is the statement of Proust's law?

It is read that when several chemical reagents are joined to form a substance, the proportion is respected, one of them being fixed and the other variable in its constitution. It was enunciated in 1799 as "Law of Definite Proportions. It is clearly observed that in such an equation the proportions are respected.

Where it is possible to calculate which quantity is formed, who limits and who exceeds in the reaction. Only the necessary amount of reagent participates in contrast to the one that limits the mass. In the same way the law asserts that the pure elements remain in a constant proportion.

Proust's Law experiment

The study of a sample of water, consisting of hydrogen and oxygen. The test is to calculate the molecular weight for each element involved in the reaction. So for the ratio according to the equation 1: 1, the amount of hydrogen is 2 gr, of oxygen 16 gr, for a product of 18 g. For the ratio according to the equation 2: 1.

The amount of hydrogen is 8 g, of oxygen is 64 g and of product 72 g. The general proportion is made with the sum of the mass quantities of the elements. For hydrogen 10 gr, oxygen 80 gr and the product 90 gr. Then the general ratio of the reaction is 1: 8. You can be more specific by calculating the percentage of the reagents.

Where the ratio of atomic weight and molecular weight per hundred is established. Obtaining that for hydrogen it is 11.11% and for oxygen it is 88.88% by mass combined in the same proportion already described above.  I conclude that water is a pure substance. That in both experiences it is asserted that the surface of the water is fixed.

What does the law of definite proportions say?

This law holds that first when two or more elements combine to form a compound, the defined proportions are kept, second it states that a chemical compound is always formed by the same mass ratio of the elements involved.

Examples of Proust's law

Proust law
Proust law


Analyze carbon dioxide (CO2), explaining based on Proust's law. For the reaction C +2 + 02 -- CO, one mole of carbon is 12 gr, of oxygen is 16 gr, for a total of 28 g of product, the ratio of the equation is 1: 2. While for the reaction C +4 +O2-- CO2, for carbon it is 24 gr, that of oxygen is 48 gr, the product will have 72 g.

the ratio of the equation is 1:2. So looking for the least common multiple of 24 and 48 gives us the general ratio of the equations is 3: 8. In conclusion, the element with the highest atomic mass has the most participation in this case oxygen, it is worth remembering that it is combined with the statement of the law of definite proportions.

When he expresses that the elements combine to form a compound, they do so in fixed proportions.


If iron and sulfur are joined in the ratio 1.75: 1 and 3 grams of iron and 3 grams of sulfur are reacted, what element is left over, in what quantity and how much product is formed?According to the chemical equation iron has a ratio of 1.75 and combines 3 gr while for sulfur.

Its ratio is 1 and 3 gr is combined. Then we look for the amount of sulfur that reacts using the conversion factor, 3 gr Fe = 1 gr S /1.75 gr Fe. The gr of S = 1.71. To find the amount of sulfur left over we say: 3 gr S = 1.71 gr = 1.29 g. For the amount of product formed: 3 gr Fe + 1.71 gr S = 4.71 gr of calcium sulfide.

The answer is, the law of proportions defined is very precise and is fulfilled in this example. 3 g of iron reacted with 1.71 g of sulfur to form 4.71 g of iron sulfide, 1.29 g of sulfur remained.


Compound A is placed to react with compound B. compound A is the nitrogen element and compound B is the oxygen element. What will be the proportion of the reaction? What is the centesimal composition of each element involved in the reaction?. Taking into account the chemical equation we have to make the stoichiometry relationship.

The atomic weight of nitrogen is 14 x 2 = 28, oxygen its atomic weight is 16 x 3 = 48. Looking for the least common multiple of 28 and 48 we have that is 3. Answer, oxygen is in greater proportion 3, participates more, while nitrogen limits the reaction. The ratio is 1:3.Looking for the centesimal composition of the reagents we proceed to: The calculation of the molecular weight of the product.

For the molecular weight. Pm (NO)= N: 14 X 1 = 14. For O = 16 X 1 = 16, Total Pm = 30. Calculating percentage with N= 14 /30 X 100. N = 46.66 %. For the O= 16/30, O= 53.33 %.Answer, the proportion is confirmed because oxygen has a greater amount. The relationship of masses, atomic weight, molecular weight per hundred, which has to do with the conservation of masses is evidenced.

Proust's law is interesting, how fine!, easy to understand. Check this article well and if you forget something, open it again.

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