Molecular Interpretation of Raoult' s law We have to consider the rate at which molecules leave and enter solution to understand raoult's law The presence of a second component actually hinders molecules from leaving the solution, but does not inhibit the rate at which they return The rate at which a molecule leave the surface is proportional to the number of them at the surface which is proportional to their mole fraction rate of vapourization where k is a proportionality constant. the rate at which molecules return is proportional to their gas phase concentration, which is proportional their partial pressure rate of condensation =kpa At equilibrium, rates of vapourization and condensation are equal, so k A k For a pure liquid, XA=1, So PA k k