2022 O Level Chem P1 Q8 — Stoichiometry / Hydrated Salts Solution

Answer: D. Find the total mass of \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\) first, then the mass of all the oxygen atoms in one formula unit.

Relative molecular mass: \(M_r = 64 + 32 + 4 \times 16 + 5 \times 18 = 64 + 32 + 64 + 90 = 250\).

Oxygen atom count per formula unit: 4 from \(\text{SO}_4\) + 5 from \(5 \times \text{H}_2\text{O}\) (1 O per water) = 9 oxygens. Mass of oxygen = \(9 \times 16 = 144\).

Percentage of oxygen by mass = \(\dfrac{9 \times 16}{250} \times 100\) — matching option D.

Options A and C use \(M_r = 178\), which forgets to add the mass of the five waters of crystallisation. Options A and B use \(5 \times 16\) for the oxygen mass, missing the four oxygens in the sulfate group.