Henry's law is one of the gas laws formulated by William Henry in 1803 and states as "at constant temperature, partial pressure of a gas in the vapour phase (p) is proportional to the mole fraction of the gas (X) in the solution.
P ∝ X
Where P = partial pressure of gas
X = Mole fraction of gas in solution
or, P = kH.X
Where kH is called Henry's law constant.