Law of Constant Composition — Explained with Copper Oxide Example

Statement: 'A pure chemical compound always contains the same elements in the same definite proportion by mass, regardless of its source or method of preparation.'

Proposed by: Joseph Louis Proust (1754–1826), French chemist. Also called: Law of Definite Proportions or Proust's Law.

Key points:

1. The composition (ratio of elements by mass) is always fixed for a given compound.
2. It does not matter where the compound comes from or how it is made.
3. Only pure compounds obey this law — mixtures do not.

Corollary: Dalton's atomic theory explained this law — atoms combine in fixed whole-number ratios.

Copper oxide (CuO) example:

Molar masses: • Cu = 63.55 g/mol • O = 16.00 g/mol • CuO = 79.55 g/mol

Composition by mass: • Cu: (63.55 / 79.55) × 100 = 79.89% ≈ 80% • O: (16.00 / 79.55) × 100 = 20.11% ≈ 20%

Mass ratio Cu : O = 79.89 : 20.11 ≈ 4 : 1

This ratio is constant regardless of: • Whether CuO is prepared by heating copper in air • Or by thermal decomposition of Cu(NO₃)₂ or CuCO₃ • Or obtained from natural mineral deposits (tenorite)

All samples of CuO give the same Cu:O ratio = 4:1 by mass.

Proust's experiment: Proust showed that two samples of copper carbonate — one natural, one synthetic — both gave the same ratio of elements on analysis. This disproved Berthollet's view that compounds could have variable compositions.

Significance of the law:

1. Confirms that compounds have definite chemical formulas.
2. Foundation for stoichiometry — calculating reactant/product amounts.
3. Used in empirical formula determination.
4. Supports Dalton's atomic theory — atoms of a given element are identical and combine in fixed ratios.

Limitation: Non-stoichiometric compounds (berthollides) like FeO (which can vary from Fe₀.₉₄O to FeO) are exceptions to this law.