With these assumptions, the adsorbed gasses can be described as an ideal gas mixture in equilibrium with an ideal adsorbed phase on the adsorbent surface. For a binary mixture of, say, CO 2 and N 2 , and for a given temperature, gas composition, and the total gas pressure, the number of moles adsorbed for a binary mixture is obtained by solving simultaneously the set of equations governing IAST. One of these IAST equations follows from the condition that at equilibrium, the spreading pressures of CO 2 and N 2 are equal: