Question

Cyclohexane has a freezing point of 6.50 ∘C and a Kf of 20.0 ∘C/m. What is the freezing point of a solution made by dissolving 0.540 g of biphenyl (C12H10) in 25.0 g of cyclohexane?

Answer 1

moles C₁₂H₁₀ = 0.540 g / 154.212 g/mol= 0.00350 mol
m = 0.00350 / 0.0250 Kg = 0.140 mol / kg

delta T = i K_f m

deltaT = Change in temperature in °C
i = van 't Hoff factor
K_f = molal freezing point depression constant or cryoscopic constant in °C kg/mol
m = molality of the solute in mol solute/kg solvent.
delta T = i K_f m

           = 0.140 x 20.0

           = 2.8 °C
freezing point = 6.50 - 2.8 = 3.7 °C