Calculate the percentage of water of crystallization in CuSO₄·5H₂O (H = 1, O = 16, S = 32, Cu = 64)

CuSO₄·5H₂O [H = 1, O = 16, S = 32, Cu = 64]

Molar mass: CuSO₄ = 64 + 32 + (4 × 16) = 64 + 32 + 64 = 160 5H₂O = 5 × 18 = 90 Total = 160 + 90 = 250 g mol⁻¹

Percentage of water of crystallisation = (mass of water ÷ molar mass of the hydrated salt) × 100 = (90 ÷ 250) × 100 = 36 %

Percentage of water of crystallisation = 36 %

Percentage composition is always (part ÷ whole) × 100, and the whole is the HYDRATED salt — all 250, water included. Dividing by 160 instead treats the water as though it were outside the formula and gives 56.25 %, which is impossible: a part cannot be more than half again of the whole.

The other slip is forgetting the 5. There are five water molecules, so 5 × 18 = 90, not 18.