rate₁ / rate₂ = √(M₂ / M₁)
Graham's law (1848) states that the rate of effusion or diffusion of a gas is inversely proportional to the square root of its molar mass: rate ∝ 1/√M. For two gases: rate₁/rate₂ = √(M₂/M₁). This means lighter gases move faster — hydrogen (M=2) diffuses 4× faster than oxygen (M=32) because √(32/2) = 4. The law derives from kinetic molecular theory: at the same temperature, all gases have the same average kinetic energy (½mv² = 3/2 kT), so lighter molecules must move faster. Applications include: uranium enrichment (separating ²³⁵UF₆ from ²³⁸UF₆ by gaseous diffusion), detecting gas leaks, estimating ventilation rates, and understanding how aromas spread.