Comprehension:
Colligative Properties
Vapour pressure of a solution decreases when a non-volatile solute is added to a volatile solvent. There are many properties of solutions which are connected with this decrease of vapour pressure. These are: (1) relative lowering of vapour pressure of the solvent (2) depression of freezing point of the solvent (3) elevation of boiling point of the solvent and (4) osmotic pressure of the solution. All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such properties are called colligative properties.
The osmotic pressure of a solution is directly proportional to
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → concentration of solution
Osmotic pressure (π) is the extra pressure that must be applied on the solution to stop the flow of solvent into it through a semipermeable membrane. For dilute solutions it obeys the van’t Hoff equation, which has the same form as the ideal gas equation:
\( \displaystyle \pi=CRT=\frac{n_2}{V}RT \)
Here C is the molar concentration of the solute, R the gas constant and T the absolute temperature. At a given temperature, π is therefore directly proportional to the concentration of the solution.
Why the other options are incorrect
Option (1) atmospheric pressure — Incorrect. Atmospheric pressure does not appear in π = CRT; osmotic pressure is set by the solution itself.
Option (3) volume of solution — Incorrect. For a fixed amount of solute, π = n2RT/V, so it is inversely proportional to the volume.
Option (4) Square of temperature — Incorrect. π is directly proportional to the absolute temperature T, not to T2.
NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – “… For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the solution at a given temperature T. Thus:
Π = C R T (1.39)”