Question

The unit of molal depression constant kf is

Answer 1

Answer:

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?

Strategy:

Step 1: Calculate the freezing point depression of benzene.

deltaTf = (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.

deltaTf = (Kf) (m)

(2.7 oC) = (Kf) (0.625 m)

Kf = 4.3 oC/m.