Which law describes the rate of effusion being inversely proportional to the square root of the molar mass?

• A. Charles' Law
• B. Graham's Law
• C. Boyle's Law
• D. Avogadro's Law

Answer: (B)

The rate of effusion is governed by how fast the gas molecules move. Lighter molecules move faster at a given temperature, so they effuse more quickly. This is captured by Graham's law: the rate of effusion is inversely proportional to the square root of the molar mass. In practice, the ratio of effusion rates for two gases equals the square root of the inverse ratio of their molar masses: rate1/rate2 = √(M2/M1). This ties directly to the kinetic-molecular picture where the average speed ∝ 1/√M. For example, a light gas like helium will effuse much faster than a heavy gas like xenon. The other laws describe how pressure, volume, and temperature relate under different conditions rather than effusion rates, so they don’t describe this effect.