Question

A 5% solution (w/v) of cane-sugar (Mol. weight= 342) is isotonic with 0.877% (w/v) of urea solution. Find molecular weight of urea (round off to the nearest integer).

Answer 1

When two solutions are isotonic, they have equal osmotic pressures.
But osmotic pressure is directly proportional to the concentration. Hence, the two solutions have equal value of concentrations.
Hence, $\frac{\text{mass of cane sugar}}{\text{molar mass of cane sugar}}=\frac{\text{mass of urea}}{\text{molar mass of urea}}$.
Hence, $\frac{5}{342}=\frac{0.877}{\text{molar mass of urea}}$.
Hence, the molar mass of urea is 60 g/mol.