A solution of urea (molar mass 60   g   mol - 1 ) boils at 100 . 20 ° C at the atmospheric…

We know that whenever a non-volatile solute is added to a liquid solvent a boiling point of solution rises while there is a depression in the freezing point of the solution.

Now,

$∆{\mathrm{T}}_{\mathrm{b}} = {\mathrm{K}}_{\mathrm{b}}\mathrm{m}$

$∆{\mathrm{T}}_{\mathrm{b}} = 100.20-100 = 0.{20}^{°} \mathrm{C}$

So, molality of solution will be 

Molality $=$ $\frac{0.20}{1.86} \mathrm{m}$

Thus, for freezing temperature

$$\begin{gathered} ∆{\mathrm{T}}_{\mathrm{f}} = {\mathrm{K}}_{\mathrm{f}}\times \mathrm{m} \\ ∆{\mathrm{T}}_{\mathrm{f}} = 1.86\times \frac{0.20}{1.86} \\ ∆{\mathrm{T}}_{\mathrm{f}} = 0.{726}^{°} \mathrm{C} \\ 0-\mathrm{T} = 0.{726}^{°} \mathrm{C} \\ \mathrm{T} = -0.{726}^{°} \mathrm{C} \end{gathered}$$