Law of conservation of mass
According to the law of conservation of mass, during chemical reaction matter is neither created nor destroyed.
Thus in a chemical reaction, total mass of the products is equal to the total mass of the reactants.
Law of constant proportions
The law of constant proportions is also known as the law of definite proportions.
This law was postulated by Proust in 1797. “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).
Carbon dioxide gas (CO2) is always found to contain carbon and oxygen combined in the ratio of 3:8 by mass.