Question

The vapour pressure of a solvent decreased by 10 mm 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 20 mm of mercury?

• A. 0.8
• B. 0.6
• C. 0.4
• D. 0.2

Answer 1

Answer: (B)

0.6

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 mole fraction of solute = 0.4

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

Hence, the correct option is $B$