Question

Addition of a non-volatile solute causes lowering in vapour pressure of a solvent from $0.8$ atm. to $0.2$ atm. What is the mole fraction of solvent?

• A. 0.25
• B. 0.75
• C. 0.50
• D. Cannot be predicted

Answer 1

Answer: (A)

From Raoult's law, $\frac{P-P_s}{P}=$ mole fraction of solute $\left(x_A\right)$
$\therefore x_A=\frac{0.8-0.2}{0.8}=\frac{0.6}{0.8}=0.75$
Hence, mole-fraction of solvent $=1-0.75=0.25$