The law of constant proportion was given by Lavosier.
It tells that in a chemical reaction compound always contain the same elements present in definite proportion by mass

The Law of Constant Proportion

The law of constant proportion, also known as the law of definite proportions, was given by the French chemist Joseph Louis Proust in the late 18th century. It is one of the fundamental laws of chemistry that helps us understand the composition of compounds.

What does it mean?

The law of constant proportion states that in a given chemical compound, the elements are always present in fixed and definite proportions by mass. This means that the ratio of the masses of the constituent elements in a compound is always the same, regardless of the source or method of preparation of the compound.

Explanation

Let's take an example to understand the law of constant proportion more clearly. Consider water (H2O), a compound made up of hydrogen and oxygen. According to the law of constant proportion, the ratio of the mass of hydrogen to the mass of oxygen in water will always be the same.

If we take two samples of water from different sources or prepared by different methods, and analyze the composition of each sample, we will find that the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. This means that for every 1 gram of hydrogen in water, there will always be 8 grams of oxygen.

Similarly, if we consider another compound, carbon dioxide (CO2), the ratio of the mass of carbon to the mass of oxygen will always be the same. Regardless of the source or preparation method, the ratio of the masses will always be 12:32.

Implications

The law of constant proportion has several implications in chemistry:

1. It helps in determining the empirical formula of a compound. The empirical formula gives the simplest whole-number ratio of the atoms present in a compound.

2. It supports the atomic theory, which states that elements combine in fixed proportions to form compounds.

3. It highlights the concept of conservation of mass. Since the ratio of masses in a compound is constant, the total mass of the compound remains the same during chemical reactions.

4. It allows for the calculation of the molecular formula of a compound if the molar mass is known.

Conclusion

The law of constant proportion, given by Joseph Louis Proust, states that the elements in a compound are always present in fixed and definite proportions by mass. This law has significant implications in chemistry and helps in understanding the composition of compounds. It supports the atomic theory, aids in determining empirical and molecular formulas, and reinforces the concept of conservation of mass.