WAEC Chemistry 2016 Paper

Concept:Graham’s law of diffusion states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.Therefore, if two gases diffuse at the same rate, their molar masses must be equal.Explanation:Calculate the molar mass of propane, $\mathrm{C_3H_8}$:$$3(12.0) + 8(1.00) = 36.0 + 8.0 = 44.0$$Calculate the molar mass of carbon(IV) oxide, $\mathrm{CO_2}$:$$12.0 + 2(16.0) = 12.0 + 32.0 = 44.0$$Both propane and carbon(IV) oxide have the same relative molecular mass of $44.0$.According to Graham’s law, gases with equal relative molecular masses diffuse at the same rate under the same conditions.Hence, the reason is that their relative molecular masses are equal.Answer:C. They have the same relative molecular mass