Avogadro’s law. Gas mixtures

Avogadro’s law (1811): under the same conditions (same pressure and temperature) equal volumes of different gases contain equal numbers of molecules. Under normal conditions (0 °C, 101.325 kPa) 1 mol of any gas occupies 22.4 L — the molar volume of gases Vₘ = 22.4 L/mol — so n = V/Vₘ and V = n·22.4. The ratio of the masses of two gases in equal volumes equals the ratio of their molar masses; the density of the first gas relative to the second is D = M₁/M₂, from which M = 2·D(H₂) and M = 29·D(air), since the average molar mass of air is about 29 g/mol. In reactions between gases the coefficients also give the ratio of volumes: in H₂ + Cl₂ = 2HCl the volumes are 1 : 1 : 2. Molecules of the common simple gases are diatomic (H₂, N₂, O₂, F₂, Cl₂), ozone O₃ is triatomic and noble gases are monatomic (He, Ne, Ar). In a gas mixture the volume fraction of component i is φ = Vᵢ/V(mixture) (or ·100 % for a percentage).