Calculation of Molal Freezing Point Depression Constant
1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of benzene. The freezing point of pure benzene
is 5.5 oC, and the freezing point of the mixture is 2.8 oC. What is the molal freezing point depression constant, Kf of benzene?
- Step 1: Calculate the freezing point depression of benzene.
Tf = (Freezing point of pure solvent) - (Freezing point of solution)
(5.5 oC) - (2.8 oC) = 2.7 oC
- Step 2 : Calculate the molal concentration of the solution.
molality = moles of solute / kg of solvent
moles of naphthalene = (1.60 g) (1 mol / 128 g) = 0.0125 mol naphthalene
molality of solution = (0.0125 mol) / (0.0200 kg) = 0.625 m
- Step 3: Calculate Kf of the solution.
Tf = (Kf) (m)
(2.7 oC) = (Kf) (0.625 m)
Kf = 4.3 oC/m