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Ideal gas law: PV = nRT. R = 0.0821 L.atm/(mol.K) = 8.314 J/(mol.K) = 1.987 cal/(mol.K). At STP (0 C = 273 K, 1 atm): V_molar = 22.4 L/mol. Density of gas: d = PM/(RT) = M/22.4 (at STP). For mixtures: M_avg = sum(x_i M_i) = sum(n_i M_i)/sum(n_i). Dalton's law: P_total = sum(P_i); P_i = x_i P_total. Graham's law: rate of effusion proportional to 1/sqrt(M). Gay-Lussac's law of combining volumes: at same T,P, gas volumes combine in simple ratios. Avogadro's law: V proportional to n at constant T,P. Common calculations: (1) Given mass, find gas volume at STP. (2) Given gas collected over water, subtract water vapour pressure. (3) Given mixture composition, find average molar mass. (4) Eudiometry: combustion of gaseous mixture, measure volume changes.