For dilute solutions the activity of the solvent in bulk solution a 1 b ≃ ≃ 1 , and the activity coefficients ( γ γ ) are also assumed to ideal on the surface. Thus, a 2 s = X 2 s = θ θ , a 1 s = X 1 s , , and X 1 s + X 2 s = 1 , where X i are mole fractions. Re-writing the equilibrium constant and solving for θ θ yields