A 2% sucrose solution is isotonic with 1.5% solution of an unknown substance. What is the molecular mass of the unknown substance.

256.5

For isotonic solution,

π₁ = π₂

or C₁ = C₂

or \(\frac{W_1/M_1}{V_1}\) = \(\frac{W_2/M_2}{V_2}\)

for V₁ = V₂ = 100,

\(\frac{2}{342 × 100}\) = \(\frac{1.5}{M_2 × 100}\)

M2 = \(\frac{342 × 1.5}{2}\) = 171 x 1.5 = 256.5 g