Which quantity is represented by the relative lowering in vapour pressure?

Mole fraction of solute

In a solution containing non volatile solute, the lowering of the vapour pressure depends on the mole fraction of solute.

\(\frac{P0-P}{P0}\) = X_B 

where P0 is vapour pressure of pure solvent

P is vapour pressure of solution and

x_B is mole fraction of solute.