A nonvolatile solute is dissolved in benzene and the resulting solution has a vapor pressure of 17.5 torr. What is the mole fraction of the solute? (P° of benzene is 26.5 torr).

Question

A nonvolatile solute is dissolved in benzene and the resulting solution has a vapor pressure of 17.5 torr. What is the mole fraction of the solute? (P° of benzene is 26.5 torr).

Answer 1

The vapor pressure of a volatile solvent in a mixture is given by P = NP^o (Raoult's law) where N is the mole fraction of the solvent, and P^o is the vapor pressure of the pure solvent. Using the given values, N = 17.5/26.5 = 0.660.

As the sum of the mole fractions must = 1, the mole fraction of the solute is 1 - 0.660 = 0.340.