Molar Freezing Point Depression Constant

Question

A solution that contains 15.581 grams of benzene (molar mass = 78.1 g/mol) in 420g of cyclohexane freezes at a temperature of 9.50 degrees celsius below the freezing point of pure cyclohexane. calculate the molar freezing point depression constant, Kf, for cyclohexane.

Answer 1

The change (decrease) in the freezing point (∆T) is found using the equation ∆T = imK

∆T = 9.50

i = van't Hoff factor = 1 for benzene (a non electrolyte)

m = molality = moles of benzene per kg of cyclohexane =15.581 g/78.1/0.420 kg = 0.475 m

K = freezing point depression constant for cyclohexane

Solving for K we have...

K = ∆T/(i)(m) = 9.50º/(1)(0.475)

K = 20º/m