The Law of Constant Composition

• Established in 1799 by the French Chemist Joseph Louis Proust

• States that in a pure compound, the elements are always present in the same definite proportion by mass.

E.g., H2O

2.0160g (2x1.0080) of hydrogen will combine with

15.9994g of oxygen to produce

18.0154g of H2O

and

4.032 of hydrogen will combine with

31.988g of oxygen to produce

36.020g of H2O

Ratio of H:O is always 2:16 or 1:8

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