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Question

Biphenyl, C12H10, is a nonvolatile, nonionizing solute that is soluble in benzene, C6⁢H6.

At 25⁢ ∘C, the vapor pressure of pure benzene is 100.84 Torr. What is the vapor pressure of a solution made from dissolving 11.8 g of biphenyl in 26.2 g of benzene?

Answer 1

Molar mass biphenyl (C12H10) = 154 g / mole

Molar mass benzene (C6H6) = 78.1 g / mole

The vapor pressure of the solution will be the vapor pressure of pure benzene x the mole fraction of benzene in the solution. (See Raoult's Law). So, first we will find moles of each, then mole fraction of benzene, and finally, the vapor pressure of the solution.

moles C12H10 = 11.8 g x 1 mol / 154 g = 0.0766 mols biphenyl

moles C6H6 = 26.2 g x 1 mol / 78.1 g = 0.3355 mols benzene

Total moles = 0.0766 + 0.3355 = 0.4121

Mol fraction benzene = 0.3355 / 0.4121 = 0.8141

Vapor pressure of solution = 0.8141 x 100.84 torr = 82.1 torr