Roult's Law and Henry's Law

For an ideal mixture, the partial pressure is related to the concentration in the liquid phase by Roult's law which may be written as:

Pi = Pi* Xi

where,  

          Pi = partial pressure of component i in gas/vapor mixture

          Pi* = pressure of pure component i (vapor pressure)

          Xi = mole fraction of component i in liquid mixture

     It must be noted that this relation is usually found to be true only for high values of  Xi as compared to mole fraction of other components in the liquid mixture.

    For low values of Xi, a linear relation between Pi and Xi again exists, although the proportionality factor is Henry's constant (H') and not the vapor pressure of the pure material. i,e

Pi = H' Xi

     This is the Henry's Law.