A solution containing $8.6 \mathrm{g}{ \mathrm{L}}^{-1}$ of urea (molar mass $60 \mathrm{g}{ \mathrm{mol}}^{-1}$) is isotonic with a 5% solution of unknown solute. The molar mass of the solute will be

Isotonic solution has the same osmotic pressure and hence the same concentration in the solution.

$\frac{\left(8.6 \mathrm{g}/60 \mathrm{g}{ \mathrm{mol}}^{-1}\right)}{1 \mathrm{L}}=\frac{(5 \mathrm{g}/M)}{0.1 \mathrm{L}}$. Hence $M=\frac{5\times 60}{8.6\times 0.1} \mathrm{g}{ \mathrm{mol}}^{-1}=348.8 \mathrm{g}{ \mathrm{mol}}^{-1}$