Question

Cyclohexane (C6H12) freezes at 6.55 degrees C. A solution of 1.67g of carbon disulfide (CS2) in 50.0g of cyclohexane freezes at -2.55 degrees C. What is the freezing point depression constant for cyclohexane?

Answer 1

Cyclohexane (C6H12) freezes at 6.55 degrees C. A solution of 1.67g of carbon disulfide (CS2) in 50.0g of cyclohexane freezes at -2.55 degrees C

To calculate the freezing point depression constant for cyclohexane use this equation...
dTfp = Kf x m x i

Here...
dTfp = change in freezing point (fp) = fp pure solvent - fp solution

= 6.55°C – (- 2.55)°C = 9.1°C
Kf = the constant for the solvent
m = molality = moles solute / kg solvent

i = van't hoff factor = 1 for CS2

Moles CS2 = 1.67g / 76 g/mol = 0.0220 Moles of CS2
m = 0.0220 g / 0.0500 Kg = 0.439 m

Now put the values in the following expression:

dTfp = Kf x m x i

9.1 °C = Kf x 0.439 m x 1

Kf = 20.73 °C /m