Henry’s Law
Henry’s law states that the partial pressure of a solution component becomes proportional to its mole fraction as the mole fraction approaches zero.
This is diplayed graphically below in the plot of vapor pressures of components A and B. The curves approach the limiting slopes kA and kB as the concentration of the component approaches zero. As the concentration of component approaches zero each molecule of that component is completely surrounded by the solvent. Thus ki represents the vapor pressure that species i has in this environment.
As an approxmate law applied to the solubility of gases in liquids it is given as
where P2 is the vapor pressure of the minor component (2) (a solute) in a dilute solution. For oxygen dissolved in water k2 is 0.426 x 105 atm. At one atmosphere the solubility in mole fractions of O2 in water is 1/(0.426 x 105) = 2.35 x 10-5. To find the molarity of O2 solubility in water we multiply the mole fraction by the molarity of water (55.5M) to get 1.30x 10-3M. To calculate the molarity of O2 in water at 25°C we multiply the solubility by 0.2 as air is 20% O2.to get 2.61x10-4M.