Question

The freezing point of biphenyl (molar mass 154g/mole) is 71.0 degrees. When 1.240 g of benzophenone (molar mass 182 g/mole) is added to 9.52 g of biphenyl the solution is found to freeze at 14.3 degrees Celsius. Calculate the freezing point depression constant, Kf, for biphenyl.

Answer 1

Calculate the freezing point depression of biphenyl.

T_f = (Freezing point of pure biphenyl) - (Freezing point of mixture)
(71.0 ^oC) - (14.3 ^oC) = 56.7 ^oC

Calculate the molal concentration of the mixture.

molality = moles of solute / kg of solvent
moles of benzophenone = (1.24 g/182 g/mol) = 0.0068 mol benzophenone
molality of solution = (0.0068 mol) / (0.00952 kg) = 0.714 m

Calculate K_f of the solution.

T_f = (K_f) (m)
(56.7 ^oC) = (K_f) (0.714 m)
K_f = 79.411 ^oC/m