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.”

For Example:

Water (H₂O) always contains hydrogen and oxygen in a ratio :

Hydrogen Oxygen

2g 16g

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.

Similarly,

Ammonia (NH₃) contains nitrogen and hydrogen:

Nitrogen Hydrogen

14g 3g

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