According to Raoult, the relative lowering of vapour pressure of a solvent over a solution containing a non-volatile solute is equal to:

1. From $p_1 = x_1 p_1^0$, the lowering is $\Delta p_1 = p_1^0(1 - x_1) = x_2 p_1^0$. 2. Dividing by $p_1^0$: $(p_1^0 - p_1)/p_1^0 = x_2$. 3. The left side is the relative lowering of vapour pressure and equals $x_2$, the mole fraction of the solute. _Source: NCERT Class 12 Chemistry Ch 1 "Solutions", p.15_