A 5% solution of cane sugar (molecular weight = 342) is isotonic with a 1% solution of substance X. The molecular weight of X is

5% of cane sugar means 5 g is present in 100 ml of solution and 1% means 1 g of X present in 100 ml of solution.

Isotonic solutions: Same osmotic pressure (i.e., same C_effective)

$$\begin{gathered} \frac{5}{342\times {\displaystyle \frac{100}{1000}}}=\frac{1}{\mathrm{molar} \mathrm{mass} \mathrm{of} \mathrm{X}\times {\displaystyle \frac{100}{1000}}} \\ \mathrm{Molar} \mathrm{mass} \mathrm{of} \mathrm{X}=68.4 \end{gathered}$$