Henry's Law is concerned with the solubility of a gas in a liquid to form an ideal dilute solution. An ideal dilute solution is defined as one in which the Activity Coefficient of the solvent is unity. This will clearly be so at infinite dilution, but in practice, it often remains essentially unchanged during the addition of small amounts of solute, say a few mole per cent, and is therefore an adequate approximation for real dilute solutions.
If the nonideality of the gas phase can be ignored, i.e., the pressure is sufficiently low, we find that the solubility of the gas in the liquid is directly proportional to its partial pressure in the gas phase. If the volatility of the solvent is low, this can be approximated as the total pressure and this is the form in which the Law is most usually used.
Henry's constant is usually defined as:
where is the mole fraction of solute gas in the liquid phase at pressure P. H is, of course, a function of temperature.