JAMB Chemistry 2022 Paper

Concept:Dilution keeps the amount of solute constant, so $C_1V_1 = C_2V_2$.Find the final volume $V_2$, then subtract the original acid volume to get the water added.Explanation:Given that $C_1 = 2.0\ \mathrm{mol/dm^3}$ and $V_1 = 10\ \mathrm{dm^3}$.The final concentration is $C_2 = 0.5\ \mathrm{mol/dm^3}$.Using the dilution formula:$$C_1V_1 = C_2V_2$$$$2.0 \times 10 = 0.5 \times V_2$$$$20 = 0.5V_2$$$$V_2 = \frac{20}{0.5} = 40\ \mathrm{dm^3}$$This $40\ \mathrm{dm^3}$ is the total final volume of the solution.Since the original acid already occupies $10\ \mathrm{dm^3}$, the water to be added is:$$40 - 10 = 30\ \mathrm{dm^3}$$Answer:The volume of water added is $30\ \mathrm{dm^3}$, so the correct option is A.