The weight of urea to be dissolved in 100 g. of water to decrease the vapour pressure of water by 5% is

Given, Wt. of urea(solute)=xg ; Wt. of solvent(water)=100gm

 Lowering of V.P (LVP)=5%

RLVP=$\frac{5}{100}=0.05$

$$\begin{gathered} {\mathrm{n}}_{\mathrm{s}}=\frac{{\mathrm{W}}_{\mathrm{s}}}{60}=\frac{\mathrm{x}}{60} ; {\mathrm{n}}_{\mathrm{o}}=\frac{100}{18} \\ \mathrm{RLVP}=\frac{{\mathrm{n}}_{\mathrm{s}}}{{\mathrm{n}}_{\mathrm{s}}+{\mathrm{n}}_{\mathrm{o}}}=\frac{{\mathrm{n}}_{\mathrm{s}}}{{\mathrm{n}}_{\mathrm{o}}} \\ 0.05=\frac{\mathrm{x}/60}{100/18} \\ \mathrm{x}=\mathrm{Wt}. \mathrm{of} \mathrm{solute}=16.66\mathrm{g}. \end{gathered}$$