What is the Law of constant proportions ?

-:Law of constant proportions:- 

The law of constant proportions  also known  the law of definite proportions. 

The law of constant proportions was postulated by Proust in 1797. According to this law, "A chemical compound always consists of the same elements combined together in the same ratio, independent of the method by which it is prepared, or the source from where it is obtained." 

For example, water (H2O) is always found to contain only hydrogen and oxygen. The ratio in which hydrogen and oxygen are present in water is also found to be fixed at 1 :8 by mass (or 2: 1 by volume) independent of the source of water. 

Thus, if we decompose 9.0g water, 1.0 g of hydrogen and 8.0 g oxygen are obtained always independent of the source of water. 

Carbon dioxide gas (CO2) is always found to contain carbon and oxygen combined in the ratio of 3:5 by mass. Thus, if 3.0 g of carbon is burnt, it combines with 8.0 g of oxygen to give 11.0 g of carbon dioxide.