A solution containing 10 g per (dm^3) of urea (molecular mass (=60...

The molar concentration of urea solution =\frac{10 g / d m^3}{\text{ Mol. wt. of urea }}=\frac{10}{60} \mathrm{M}=\frac{1}{6} \mathrm{M}Molar concentration of $5 \%$ nonvolatile solute $$\begin{aligned}& =\frac{50 \mathrm{~g} / \mathrm{dm}^3}{\text{ mol. wt. of non - volatile solute }}\\ & =\frac{50}{\mathrm{~m}} \mathrm{M} \end{aligned}$$Both solutions are isotonic to each other; therefore, $\quad \frac{1}{6}=\frac{50}{m}$ m=50 $\times$6=300 $\mathrm{~g} \mathrm{~mol}^{-1}$Caution $10 \mathrm{~g}$ per $\mathrm{dm}^3$ of urea is isotonic with $5 \%$ solution of a non-volatile solute. Hence, osmosis cannot occur between the two solutions. Thus, their molar concentrations are equal to each other.