The molecular mass of sodium sulphate $(\mathrm{Na}_2\mathrm{SO}_4)$ = 2 × 23 + 32 + 4 × 16 = 46 + 32 + 64 = 142 amu or 142 u Mass % of sodium = $\frac{2\times 23}{142}$ × 100 = $\frac{46}{142}$ × 100 = 32.39 % Mass % of sulphur = $\frac{32}{142}$ × 100 = 22.53 % Mass % of oxygen = $\frac{4\times 16}{142}$ × 100 = 45.07 %