WAEC Chemistry 2013 Paper

Concept:Percentage by mass of an element is found by dividing the mass of that element in one mole of the compound by the molar mass of the compound, then multiplying by 100.Explanation:The compound is $\mathrm{Ca(OCl)_2}$.It contains 1 calcium atom, 2 oxygen atoms, and 2 chlorine atoms.Mass of calcium in one mole $= 40.0$ gMass of oxygen $= 2 \times 16.0 = 32.0$ gMass of chlorine $= 2 \times 35.5 = 71.0$ gMolar mass of $\mathrm{Ca(OCl)_2} = 40.0 + 32.0 + 71.0 = 143.0$ g/molPercentage by mass of calcium $= \frac{40.0}{143.0} \times 100\%$$= 27.97\% \approx 28.0\%$Answer:A. 28.0%