WAEC Chemistry 2020 Paper

Concept:The number of moles of a solute is found by multiplying its molar concentration by its volume in cubic decimetres ($\mathrm{dm}^3$).Explanation:First, convert the given volume from $\mathrm{cm}^3$ to $\mathrm{dm}^3$.Since $1\ \mathrm{dm}^3 = 1000\ \mathrm{cm}^3$, we have:$$50\ \mathrm{cm}^3 = \frac{50}{1000}\ \mathrm{dm}^3 = 0.05\ \mathrm{dm}^3.$$Now apply the formula:$$\text{Moles of } \mathrm{H}_2\mathrm{SO}_4 = \text{Concentration} \times \text{Volume in } \mathrm{dm}^3.$$Substitute the given values:$$\text{Moles} = 0.108\ \mathrm{mol}\ \mathrm{dm}^{-3} \times 0.05\ \mathrm{dm}^3.$$Perform the multiplication:$$\text{Moles} = 0.0054\ \mathrm{mol}.$$Writing this in standard form gives:$$0.0054 = 5.4 \times 10^{-3}.$$Answer:There are $5.4 \times 10^{-3}$ moles of $\mathrm{H}_2\mathrm{SO}_4$.The correct option is B.