Question

The freezing point of nitrobenzene is 278.8 K. A 0.25 molal solution of a substance (mol. wt. = 120) in nitrobenzene has a freezing point of 276.8 K. Calculate the molal depression constant of nitrobenzene.

Answer 1

Given: Freezing point of pure nitrobenzene, `T_f^0` = 278.8 K

Freezing point of solution, T_f = 276.8 K

Depression in freezing point,

ΔT_f = 278.8 − 276.8

= 2.0 K

Molality, m = 0.25 mol/kg

We know that 

ΔT_f​ = K_f ​⋅ m

∴ `K_f = (Delta T_f)/m`

= `2.0/0.25`

= 8.0 K kg mol⁻¹