PhysChemBasics#2 Gases 101 The state of an ideal gas can be calculated using the ideal gas law. The composition of gas mixtures can be given using Dalton's partial pressures. Microscopically a gas can be described by Maxwell-Boltzmann theory with the parameters average speed and mean free path being most important. Deviatons from the ideal behavior can be quantified using the van-der-Waals equation, which explains why at the critical point a gas behaves only "3/8 idealy".