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.$\frac{atm}{mol.K}$ or 8.314 $\frac{J}{mol.K}$). Density of gas: d = $\frac{PM}{RT}$. 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 $P_{total}$.