Law of definite proportion:
This law was put forward by French chemist Joseph Louis Proust in 1799. It states:
According to the law of definite proportion “same compounds are always consisting of same elements combined in a same fixed ratio by mass.”
Water (H2O) always contains hydrogen and oxygen in a ratio :
If we analyze samples of water obtained from different sources (e.g sea, spring, rain or produced in a reaction), we will find that water from all these sources consist of 11.11% Hydrogen and 88.89% Oxygen.
Ammonia (NH3) contains nitrogen and hydrogen: