WAEC Chemistry 2012 Paper

Concept:According to Graham’s law of diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.Explanation:Graham’s law is written as:$$r \propto \sqrt{\frac{1}{M}}$$Here, $r$ is the rate of diffusion and $M$ is the molar mass.So, a gas with a higher molar mass diffuses more slowly.Now calculate the molar mass of each gas:$\mathrm{Cl}_2 = 2 \times 35.5 = 71.0$ g/mol$\mathrm{H}_2 = 2 \times 1.0 = 2.0$ g/mol$\mathrm{He} = 4.0$ g/mol$\mathrm{O}_2 = 2 \times 16.0 = 32.0$ g/molSince $\mathrm{Cl}_2$ has the highest molar mass, it has the lowest rate of diffusion.Answer:A. $\mathrm{Cl}_2$