Law of Constant Composition(law of definite proportions)

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.


Ammonia (NH3) contains nitrogen and hydrogen:

Nitrogen Hydrogen

14g 3g

Also Read !!!

Law of conservation of mass

Mole and Avogadro’s Number | Important Questions

Stoichiometry Important Terms Definitions

Mole and Chemical Equation

What is the difference between Octet Rule and Duplet Rule

Basic Atomic Structure

What is the difference between Shell , Subshell and Orbital

How ions are formed (Cation vs Anion)

Ionic Bond : Definition, Examples & Formation

Covalent Bond :Definition and Examples

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