JAMB Chemistry 2020 Paper

Concept:At S.T.P., one mole of any gas occupies $22.4\ \mathrm{dm}^3$.Use molar mass to find moles and then the volume of propane.Explanation:Calculate the number of moles in $8\ \mathrm{g}$ of methane ($\mathrm{CH}_4$).Molar mass of $\mathrm{CH}_4 = 12 + 4(1) = 16\ \mathrm{g/mol}$.Moles of methane $= \frac{8\ \mathrm{g}}{16\ \mathrm{g/mol}} = 0.5\ \mathrm{mol}$.Given: $0.5\ \mathrm{mol}$ occupies $11.2\ \mathrm{dm}^3$, confirming the molar volume $22.4\ \mathrm{dm}^3$ at S.T.P.Now consider propane ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3$), which is $\mathrm{C}_3\mathrm{H}_8$.Molar mass of propane $= 3(12) + 8(1) = 36 + 8 = 44\ \mathrm{g/mol}$.Moles of propane in $22\ \mathrm{g}$ $= \frac{22\ \mathrm{g}}{44\ \mathrm{g/mol}} = 0.5\ \mathrm{mol}$.Since $0.5\ \mathrm{mol}$ of a gas at S.T.P. occupies $11.2\ \mathrm{dm}^3$, the volume of propane is $11.2\ \mathrm{dm}^3$.Answer:B. $11.2\ \mathrm{dm}^3$