A Solution of Urea (Molar mass 56 gmol^–1) boils at 100.18⁰C at the atmospheric pressure. If K_f and K_b for water are 1.86 and 0.512 kg mol^–1 respectively. The above solution will freeze at

ΔT_f​ = K_f​× molality of solution and ΔT_b​=K_b​× molality of solution

$\frac{\mathrm{\Delta }{\mathrm{T}}_{\mathrm{f} }}{\mathrm{\Delta }{\mathrm{T}}_{\mathrm{b}}} = \frac{ {\mathrm{K}}_{\mathrm{f}} }{{\mathrm{K}}_{\mathrm{b}} }$

ΔT_b​ = T₂​−T₁​ = 100.18 −100 = 0.18^oC

K_f​ for water = 1.86 K kg mol⁻¹

K_b​ for water = 0.52 K log mol⁻¹

$$\begin{gathered} \frac{∆T_f}{0.18}=\frac{1.86}{0.512} \\ ∆{\mathrm{T}}_{\mathrm{f}}={\mathrm{T}}_{\mathrm{f}}^{\mathrm{o}}-{\mathrm{T}}_{\mathrm{f}}=0.654°\mathrm{c} \\ {\mathrm{T}}_{\mathrm{f}}=\mathrm{F}.\mathrm{P} \mathrm{of} \mathrm{solution}=-0.654°\mathrm{c} \end{gathered}$$