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 ?
Correct answer: D. 0.6
- A. 0.2
- B. 0.1
- C. 0.4
- D. 0.6
Explanation
Raoult’s law gives the pressure lowering as Δp = xsolute P°; therefore, doubling the lowering from 10 to 20 mm Hg doubles the solute mole fraction from 0.2 to 0.4. The solvent mole fraction is then 1 − 0.4 = 0.6.
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