Vapour density (VD) = density of gas / density of H2 at same T, P = M/2. So M = 2 x VD. For gas mixtures: M_avg = sum(x_i x M_i) where x_i = mole fraction. Or M_avg = total mass / total moles. Ideal gas: PV = nRT (R = 0.0821 L.atm/(mol.K) or 8.314 J/(mol.K)). Density of gas: d = PM/(RT). At STP: d = M/22.4 g/L. For reactions involving gases: use PV = nRT or volume ratios at same T, P. Dalton's law: P_total = sum(P_i), where P_i = x_i x P_total.