In the vapour above an ideal binary solution of two volatile liquids at equilibrium, the vapour phase is always richer in:

1. The partial pressure of a component scales with its pure vapour pressure $p^0$. 2. The more volatile component has the higher $p^0$ and so contributes a larger partial pressure. 3. Since vapour mole fraction $y_i = p_i / p_{total}$, the vapour is always richer in the more volatile component. _Source: NCERT Class 12 Chemistry Ch 1 "Solutions", p.10_