Elevation in b.p of an aqueous urea solution is0.52° $\left({\mathrm{K}}_{\mathrm{b}}=0.52°{\mathrm{mol}}^{-1}\mathrm{kg}\right)$. Hence, mole-fraction of urea in this solution is

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

$\mathrm{Zm}=\frac{0.52}{0.52}=1$

No. of moles of solute (urea) = 1 

Wt of solvent = 1 kg = 1000 g

No. of moles of solvent $\frac{1000}{18}=55.55$

${\mathrm{X}}_{\mathrm{Urea}}=\frac{1}{1+55.55}$

=0.018