A 5 % solution of cane sugar (molar mass: 342 g mol ^-1) is isotonic with 1 % of a solution of an unknown solute. The molar mass of unknown solute in g/mol is

Isotonic solutions have identical osmotic pressure and hence identical concentrations, i.e. the same amounts in the fixed volume of solutions. Thus,

$\frac{5\mathrm{g}}{342\mathrm{g}{\mathrm{mol}}^{-1}}=\frac{1\mathrm{g}}{M}$; This gives $M=\left(\frac{342}{5}\right)\mathrm{g}{\mathrm{mol}}^{-1}=68.4\mathrm{g}{\mathrm{mol}}^{-1}$