A solution of urea (mol. mass 56 ~g ~mol^ -1 ) boils at 100.18^ C...

$$\begin{aligned} \because \Delta T_f & =k_f \times \text{ Molality\text{ of } solution } \\ \Delta T_b & =k_b \times \text{ Molality of solution }\end{aligned}$$or \frac{\Delta T_f}{\Delta T_b}=\frac{k_f}{k_b}$Given that$\Delta T_b=T_2-T_1=100.18-100=0.18 $$\begin{aligned} & k_f \text{\text{ for } water }=1.86 \mathrm{ K} \mathrm{ kg} \mathrm{ mol}^{-1} \\ & k_b \text{ for water }=0.512 \mathrm{ K} \mathrm{ kg} \mathrm{ mol}^{-1}\end{aligned}$$ \therefore \frac{\Delta T_f}{0.18}=\frac{1.86}{0.512} $$\begin{aligned} \Delta T_f & =\frac{1.86 \times 0.18}{0.512} \\ & =0.6539 \sim 0.654 \\ \Delta T_f & =T_1-T_2 \\ 0.654 & =0^{\circ} \mathrm{C}-T_2\end{aligned}$$ \therefore T_2=-0.654^{\circ} \mathrm{C} ($T_2 \rightarrow$ Freezing point of aqueous urea solution)$