If absorption is a physical process not accompanied by any other physical or chemical process, it usually follows the Nernst distribution law: 
:"the ratio of concentrations of some solute species in two bulk phases when it is equilibrium and in contact is constant for a given solute and bulk phases":
The value of constant K depends on temperature and is called partition coefficient. This equation is valid if concentrations are not too large and if the species "x" does not change its form in any of the two phases "1" or "2". If such molecule undergoes association or dissociation then this equation still describes the equilibrium between "x" in both phases, but only for the same form – concentrations of all remaining forms must be calculated by taking into account all the other equilibria.
In the case of gas absorption, one may calculate its concentration by using, e.g., the Ideal gas law, c = p/RT. In alternative fashion, one may use partial pressures instead of concentrations.