Diethyl ether has a vapor pressure of 400.0 torr at 18°c. when a sample of benzoic acid is dissolved in ether, the vapor pressure of the solution is 342 torr. what is the mole fraction of benzoic acid in the solution?
a) 0.0169
b) 0.0197
c) 0.145
d) 0.855 e.

c) 0.145

Explanation:

By Raoult's Law, the pressure of a solution is equal to the molar fraction of the solvent multiplied by its vapor pressure. The solvent here is ether, so

342 = x*400

x = 342/400

x = 0.855

The solution has only ether and benzoic acid, so the molar fraction of benzoic acid will be

1 - 0.855 = 0.145