Law of Constant Proportion states that a chemical compound always contains exactly the same proportion of elements by mass.
This law is also known as Law of definite proportions. Joseph Louis Proust gave this law hence, this law is also known as Proust’* Law.
Explanation of the law:-
Compounds are formed by the combination of two or more elements. In a compound the ratio of the atoms or element by mass *** always same irrespective of the source of compound. This means a certain compound always formed by the combination of atoms in same ratio by mass. If the ratio of mass of constituent atoms will be altered the new compound is formed.
Examples:
Water is formed by the combination of hydrogen and oxygen. The ratio of masses of hydrogen and oxygen is always in 1:8 in water irrespective of source of water. Whether you collect the water from a well, river, pond or from anywhere the ratio of their constituent atoms by mass will always same.
Nitrogen dioxide is a compound, which is formed by the combination of nitrogen and oxygen.
The ratio of nitrogen and oxygen by mass in nitrogen dioxide is in 7:16.
Nitrous oxide is a compound which is also formed by the combination of nitrogen and oxygen.
The ratio of nitrogen and oxygen in nitrous oxide is in 28:16.
Nitric oxide is a compound, which is also formed by the combination of nitrogen and oxygen.
The ratio of nitrogen and oxygen in nitric oxide is in 7:8.
From the above three examples it is clear that if the ratio of the atoms by mass is altered then the new compound is formed, such as in the case of nitrogen dioxide, nitrous oxide, nitric oxide. These three compounds are formed by the combination of same atoms but because of combination of the constituent atoms in different ratios by mass new compound is formed.
