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. If gas X diffuses twice as fast as gas Y, what is the ratio of the molar mass of X to that of Y?

By Graham's Law: r_X/r_Y = √(M_Y/M_X). Given r_X/r_Y = 2, so 2 = √(M_Y/M_X), squaring both sides gives 4 = M_Y/M_X, which means M_X/M_Y = 1/4, i.e., the ratio is 1:4. A common error is forgetting to square the rate ratio, giving the wrong answer of 1:2.