WAEC Chemistry 2013 Paper

Concept:Graham's law of diffusion states that the diffusion rate $r$ of a gas is inversely proportional to the square root of its molar mass: $r \propto \frac{1}{\sqrt{M}}$. A lighter gas therefore diffuses faster than a heavier gas.Explanation:Increasing order of diffusion rate starts with the slowest gas and ends with the fastest gas. Because molar mass determines speed, we first calculate the molar mass of each gas in the options.For sulfur dioxide: $M(\mathrm{SO}_2) = 32 + 2(16) = 64$.For oxygen: $M(\mathrm{O}_2) = 2(16) = 32$.For ammonia: $M(\mathrm{NH}_3) = 14 + 3(1) = 17$.For hydrogen: $M(\mathrm{H}_2) = 2(1) = 2$.Since diffusion rate is higher for lower molar mass, the required sequence is: $\mathrm{SO}_2$ (slowest), then $\mathrm{O}_2$, then $\mathrm{NH}_3$, then $\mathrm{H}_2$ (fastest).Comparing with the choices, only Option A presents this exact order.Answer:Option A: $\mathrm{SO}_2, \mathrm{O}_2, \mathrm{NH}_3, \mathrm{H}_2$.