State Law of Definite Proportion

The law of definite proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, no matter how or where it is prepared.

For example, water (H₂O) always has hydrogen and oxygen in a 1:8 mass ratio. Whether the water is from a river, ocean, or lab, the ratio does not change.

• The composition of a compound is constant.
• The law was given by Joseph Proust in 1799.
• Supports the idea of atoms combining in fixed proportions.

Examples

1. Carbon dioxide (CO₂) always has carbon and oxygen in 12:32 mass ratio.
2. Ammonia (NH₃) always has nitrogen and hydrogen in 14:3 mass ratio.