The ratio of the rates of effusion of two gases is inversely proportional to the square root of their molar masses. This statement is known as which law?

• A. Dalton's Law
• B. Graham's Law of Diffusion
• C. Graham's Law of Effusion
• D. Ideal Gas Law

Answer: (C)

Graham's law of effusion states that at the same temperature, the rate at which a gas passes through a small opening (effuses) is inversely proportional to the square root of its molar mass. This comes from the kinetic theory idea that all gases at the same temperature have the same average kinetic energy, so lighter molecules move faster. Since the effusion rate depends on how often molecules reach the opening, it scales with their speed, which goes as v ∝ 1/√M. Therefore, the ratio of effusion rates for two gases is r1/r2 = √(M2/M1). This is the reason the statement matches Graham's law of effusion. For contrast, Dalton's law relates to partial pressures, the Ideal Gas Law connects P, V, n, and T, and Graham's law of diffusion describes diffusion through a space rather than through a small opening, though its form is similar.