Chemistry - Liquid Solution Question with Solution | TestHub

Let the vapour pressure of the pure solvent be $P^0$.

The decrease in vapour pressure is given as $20\%$, so the lowering of vapour pressure is $\Delta P=0.20P^0$.

According to Raoult's law, the relative lowering of vapour pressure is equal to the mole fraction of the solute, $X_{\text{solute}}$.

$\frac{\Delta P}{P^0}=X_{\text{solute}}$

$X_{\text{solute}}=\frac{0.20P^0}{P^0}=0.20$

For a binary solution, the sum of mole fractions of solute and solvent is $1$.

$X_{\text{solvent}}=1-X_{\text{solute}}$

$X_{\text{solvent}}=1-0.20=0.8$

Answer: $0.8$