Which of the following colligative property is used to calculate the…

The correct answer is Option (4) → Osmotic pressure

All four colligative properties depend on the number of solute particles and can give a molar mass. For a dissolved solute, osmotic pressure is π = CRT = (n/V)RT, so

    \( \displaystyle M_2 = \frac{w_2 R T}{\pi V} \)

The osmotic pressure method is chosen for biomolecules and polymers because the measurement is made at room temperature (biomolecules are not stable at higher temperatures) and its magnitude is large even for very dilute solutions, which suits large molecules of poor solubility.

Why the other options are incorrect

• Option (1) Relative lowering of vapour pressure — Incorrect. For a dilute solution of a large molecule the lowering is too small to measure accurately.

• Option (2) Elevation in boiling point — Incorrect. Needs heating to the boiling point, which denatures biomolecules; ΔT_b is also tiny for large molar masses.

• Option (3) Depression in freezing point — Incorrect. ΔT_f is too small to measure for a dilute solution of a molecule with a molar mass of thousands.

NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – “… Measurement of osmotic pressure provides another method of determining molar masses of solutes. This method is widely used to determine molar masses of proteins, polymers and other macromolecules. The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. As compared to other colligative properties, its magnitude is large even for very dilute solutions. The technique of osmotic pressure for determination of molar mass of solutes is particularly useful for biomolecules as they are generally not stable at higher temperatures and polymers have poor solubility.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: