A 0.5 ~g / L solution of glucose is found to be isotonic with a 2.5...

Molarity of glucose solution $=\frac{0.5 \mathrm{~g}}{180 \times 1 \mathrm{~L}}=0.00277 \mathrm{M}$ Isotonic solution have equal molar concentrations, hence molarity of organic compound solution $=0.00277 \mathrm{M}$ $\therefore$ Molecular weight of compound $=\frac{2.5}{0.00277 \times 1}=900$