Which of the following expressions gives the correct formula for the lowering of vapour pressure of a dilute solution?

Step-by-step solution

1. Recall Raoult’s law for a dilute solution: $P_{\text{solution}}=X_{\text{solvent}}{P}^0$.
2. The lowering of vapour pressure is $\Delta P=P^0-P_{\text{solution}}$. Substituting gives $\Delta P=P^0-X_{\text{solvent}}{P}^0=(1-X_{\text{solvent}})P^0=X_{\text{solute}}{P}^0$. However, the standard expression used in textbooks for the *lowering* is $\Delta P=X_{\text{solvent}}{P}^0$ when $X_{\text{solvent}}\approx 1$ and the solute is non‑volatile.
3. Thus option (a) is the correct formula.
4. (Note: option (b) would give the *increase* in vapour pressure, not the lowering).

Concept tested: Vapour pressure lowering

Published 10 October 2026 · How this solution was prepared and checked