The vapor pressure of the solvent decreased by 10 mm Hg, when a non-volatile solute was added to the solvent. The mole fraction of the solute in the solution is 0.2. What should be the mole fraction of the solvent, if the decrease in vapor pressure of the solvent is required to be 20 mm Hg ?

Question

Accepted Answer

0.6. Raoult's law states $\Delta P = X_{solute} 	imes P^0$. If $\Delta P$ doubles to 20 mm Hg, and $P^0$ is constant, the mole fraction of solute must double from 0.2 to 0.4. The mole fraction of solvent is $1 - X_{solute}$, so it becomes $1 - 0.4 = 0.6$.