For a dilute solution of a non-volatile solute B in a solvent A, the relative lowering of vapour pressure equals:

Raoult's law for a dilute non-volatile solute gives $\dfrac{p_A^\circ - p_A}{p_A^\circ} = \chi_B$, where $\chi_B$ is the mole fraction of solute. This is a colligative property since it depends only on the *number* of solute particles, not their identity.