Given:
Mr_{r}r of MgSO4_{4}4.7H2_{2}2O = 246
Mr_{r}r of Oxygen = 16
Percentage by mass of Oxygen = Total mass of OxygenTotal mass of MgSO4⋅7H2O×100\; \frac{\text{Total mass of Oxygen}}{\text{Total mass of }\mathrm{MgSO_4}\cdot7\mathrm{H_2O}} \times 100Total mass of MgSO4⋅7H2OTotal mass of Oxygen×100
% by mass of Oxygen = 11 moles of oxygenMgSO4⋅7H2O×100\; \frac{\text{11 moles of oxygen}}{\mathrm{MgSO_4}\cdot7\mathrm{H_2O}} \times 100MgSO4⋅7H2O11 moles of oxygen×100 (4moles of oxygen + 7 moles of oxygen from 7H2_{2}2O )
= 11×16246×100\; \frac{11 \times 16}{246} \times 10024611×16×100
= 176246×100\; \frac{176}{246} \times 100246176×100
= 71.544%
= 71.5%
