Question

The vapour pressure of a solvent decreased by $10mm$ of mercury 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 the vapour pressure is to be $20mm$ of mercury?

• A. $0.8$
• B. $0.6$
• C. $0.4$
• D. $0.2$

Answer 1

Answer: (B)

$0.6$

$\text{Mole fraction of solute}=\frac{\text{lowering in vapour pressure}}{\text{vapour pressure of solvent}}$

comparing under the two conditions

$\frac{0.2}{\text{mole fraction of solute}}=\frac{10}{20}$

or, the mole fraction of solute $=0.4$

mole fraction of solvent $=(1-0.4)=0.6$

Hence, the correct option is $\text{B}$