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A solution containing 10 g dm⁻³ of urea (molecular mass = 60 g mol⁻¹) is isotonic with a 5% solution of a non-volatile solute. The molecular mass of this non-volatile solute is

1. 300 g mol⁻¹
2. 350 g mol⁻¹
3. 200 g mol⁻¹
4. 250 g mol⁻¹

Correct answer: 300 g mol⁻¹

Solution

For isotonic solutions, the osmotic pressure is the same. Using the formula π = (w × R × T) / (M × V), we equate the molar concentrations. For urea, molarity = 10/60 mol dm⁻³. For the non-volatile solute, 5% solution means 5 g in 100 mL (or 50 g in 1 L). Solving for molecular mass, M = 300 g mol⁻¹.

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