Question

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 15.9 g of biphenyl in 28.5 g of benzene?

Answer 1

Calculate mole fraction of solvent in the solution of nonvolatile solute biphenyle in benzene &

apply Rault's law relation as

p₁  = x₁ p₁^o

p₁ represents vapor pressure of solvent in solution , x₁ is mole fraction of solvent in solution ,

= moles of benzene / moles of benzene + moles of biphenyl

= ( 28.5/ 78 ) / [ (28.75 /78) + (15.9 / 154 )

= 0.7796

p₁^o = vapour pressure of pure solvent (ie. benzene in this case ) = 100.84 torr.

Therefore, vapor pressure of benzene in solution = (0.7796 x 100.84) torr

...............................................................................= 78.61 torr.