Biphenyl C12H10 is a nonvolatile nonionizing solute that is

Biphenyl, C12H10, is a nonvolatile, nonionizing solute that is soluble in benzene, C6H6. At 25 °C, the vapor pressure of pure benzene is 100.84 torr. What is the vapor pressure of a solution made from dissolving 10.8 g of biphenyl in 26.5 g of benzene?

Solution

  We need to calculate the mole fraction of benzene after adding the biphenyl (molar mass = 154.2 g/mole).

10.8 g C₁₂H₁₀ x (1 mole C₁₂H₁₀ / 154.2 g C₁₂H₁₀) = 0.070040 moles C₁₂H₁₀

26.5 g C6H6 x (1 mole C6H6 / 78.1 g C6H6) = 0.3393 moles C6H6

Total moles = moles C12H10 + moles C6H6 = 0.070040 + 0.3393 = 0.40934

mole fraction C6H6 = (moles C6H6 / total moles) = 0.3393 / 0.40934= 0.82889
0.752

What that means is that the vapor pressure will be 82.89% of the vapor pressure of pure benzene since the solution contains only 82.89 mole % benzene. The biphenyl contributes no vapor pressure (nonvolatile).

P = (x C6H6)(Po C6H6) where x C6H6 is the mole fraction of C6H6 and Po C6H6 is the vapor pressure of pure C6H6.

P = (0.82889)(100.84 torr) = 83.5852 torr.