JAMB Chemistry 2013 Paper

Concept:Dilution reduces concentration because the amount of solute stays the same while the total volume increases.Use the dilution formula $C_1V_1 = C_2V_2$.Explanation:Identify the initial conditions: $C_1 = 2.0\ \mathrm{mol\,dm^{-3}}$ and $V_1 = 2.5\ \mathrm{dm^3}$.Water added is $10.0\ \mathrm{dm^3}$, so the final volume is $V_2 = 2.5 + 10.0 = 12.5\ \mathrm{dm^3}$.The moles of HCl solute remain unchanged during dilution.Apply the dilution equation: $C_1V_1 = C_2V_2$.Make $C_2$ the subject: $C_2 = \frac{C_1V_1}{V_2}$.Substitute the known values: $C_2 = \frac{2.0 \times 2.5}{12.5}$.Simplify: $C_2 = \frac{5.0}{12.5} = 0.4\ \mathrm{mol\,dm^{-3}}$.Answer:The concentration of the final solution is $0.4\ \mathrm{mol\,dm^{-3}}$, which is option A.