Copper Oxide Prepared by Two Different Methods — Law of Definite Proportions Explained

The Law of Definite Proportions (proposed by Joseph Louis Proust in 1799) states:

'A given chemical compound always contains the same elements combined in the same fixed proportion by mass, irrespective of the method of its preparation or the source from which it is obtained.'

Also called: Law of Constant Composition

Key implication: No matter how CuO is made — whether by oxidising copper metal or by heating copper carbonate or copper nitrate — the ratio of copper to oxygen is always the same (approximately 4:1 by mass).

Method 1 — Direct oxidation of copper: 2Cu + O₂ → 2CuO Copper is heated in air/oxygen. The product is black copper(II) oxide.

Method 2 — Thermal decomposition of copper carbonate: CuCO₃ → CuO + CO₂ Heating copper carbonate (malachite) gives black copper oxide and carbon dioxide.

Method 3 — Thermal decomposition of copper nitrate: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂ Heating copper nitrate also produces black copper oxide.

All three methods produce CuO with the same composition regardless of preparation route.

Atomic masses: Cu = 63.5 g/mol, O = 16 g/mol

For CuO (copper(II) oxide): Molar mass = 63.5 + 16 = 79.5 g/mol

Mass ratio of Cu:O = 63.5 : 16 ≈ 4 : 1

Percentage composition:

• % Cu = (63.5/79.5) × 100 = 79.87% ≈ 80%
• % O = (16/79.5) × 100 = 20.13% ≈ 20%

This means in any sample of CuO, for every 4 g of copper there is exactly 1 g of oxygen. This ratio is invariant regardless of the preparation method, confirming the law.

For Cu₂O (copper(I) oxide): ratio = 127:16 ≈ 8:1 (different compound, different ratio — consistent with Dalton's Law of Multiple Proportions).

To verify the Law of Definite Proportions using copper oxide:

Experiment:

1. Prepare Sample A by heating copper in oxygen → weigh product
2. Prepare Sample B by heating copper carbonate → weigh product
3. Analyse both samples by reducing CuO with hydrogen: CuO + H₂ → Cu + H₂O
4. Measure mass of copper recovered and mass of oxygen removed (from water)

Result:

• Sample A: e.g., 0.796 g CuO → 0.635 g Cu + 0.160 g O → ratio Cu:O = 0.635:0.160 = 3.97 ≈ 4:1
• Sample B: e.g., 0.796 g CuO → 0.635 g Cu + 0.160 g O → same ratio 4:1

Both samples give the same ratio, confirming the law.

Significance:

• Confirms that compounds have definite, reproducible formulas
• Foundation of stoichiometry and chemical formulas
• Supports Dalton's Atomic Theory (atoms combine in fixed small whole-number ratios)
• Basis for % composition calculations and empirical formula determination

Limitations (exceptions):

1. Isotopic variation: Elements can have isotopes, so the mass ratio may slightly differ depending on isotopic composition (e.g., naturally occurring vs lab-synthesised samples)
2. Non-stoichiometric compounds (Berthollides): Some inorganic compounds (especially transition metal oxides, sulphides) exist with variable composition, e.g., Fe₀.₉₄₅O to FeO. These violate the law and are called non-stoichiometric or Daltonide violations.