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PV = nRT. R = 0.0821 L.atm/(mol.K) = 8.314 J/(mol.K). Kinetic theory assumptions: point particles, no forces, elastic collisions, KE proportional to T. Average KE = (3/2)kT per molecule = (3/2)RT per mole. KE depends only on T, not on gas identity. Three speeds: u_mp = sqrt(2RT/M), u_avg = sqrt(8RT/piM), u_rms = sqrt(3RT/M). Ratio: 1 : 1.128 : 1.224. All proportional to sqrt(T/M). Maxwell-Boltzmann distribution: bell-shaped curve plotting fraction of molecules vs speed. Peak at u_mp. Higher T: peak shifts right, flattens (broader). Higher M: peak shifts left (slower). Area under curve always = 1 (total probability). Graham's law: r1/r2 = sqrt(M2/M1). Dalton's law: P_total = sum(P_i). Mean free path: average distance between collisions = kT/(sqrt(2) pi d^2 P). Increases with T, decreases with P and molecular size.