The Henry's law relates the solubility of a gas with external pressure. The solubility is expressed in terms of molar concentration (moles/litre) or mole fraction.
The Henry's law states that, "The partial pressure of the gas [P] is proportional to the mole fraction [X] of the gas in the solution."
It can also be stated as, "Mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution."
P=KHX