Law of constant proportions:

This law states that “in a chemical substance, the elements are always present in definite proportions by mass”.

For example: In ammonia (NH₃), nitrogen and hydrogen are always present in the ratio of 14:3 by mass, whatever the method or source from which it is obtained.

In water, hydrogen and oxygen are always present in the ratio of 1:8 by mass, whatever the method or source from which water is obtained.