The properties of solutions which depend on the number of solute particles and are independent of their chemical identity are called colligative properties. These are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. The process of osmosis can be reversed if a pressure higher than the osmotic pressure is applied to the solution. Colligative properties have been used to determine the molar mass of solutes. Solutes which dissociate in solution exhibit molar mass lower than the actual molar mass and those which associate show higher molar mass than their actual values. Quantitatively, the extent to which a solute is dissociated or associated can be expressed by van’t Hoff factor i. This factor has been defined as ratio of normal molar mass to experimentally determined molar mass or as the ratio of observed colligative property to the calculated colligative property.
Which of the following relation represents correct relationship of osmotic pressure with concentration of the solution?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → π ∝ c
Osmotic pressure is the extra pressure that must be applied to a solution to stop solvent molecules passing into it through a semipermeable membrane. In the passage it is one of the four colligative properties, so it depends on the number of solute particles. For a dilute solution of molar concentration c at temperature T:
\( \displaystyle \pi=cRT \)
At a given temperature R and T are constant, so π is directly proportional to c: doubling the concentration doubles the osmotic pressure.
Why the other options are incorrect
Option (1) π ∝ \( \frac{1}{c} \) — Incorrect. By π = cRT the osmotic pressure rises as more solute is added; it does not fall as c increases.
Option (2) π ∝ c2 — Incorrect. In π = cRT the concentration appears to the first power, not squared.
Option (3) π ∝ \( \frac{1}{c^2} \) — Incorrect. Osmotic pressure rises with concentration; it does not fall as the square of it.
NCERT (2026-27 edition):
1.6.4 Osmosis and Osmotic Pressure – “… Osmotic pressure is a colligative property as it depends on the number of solute molecules and not on their identity. 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:”
\( \displaystyle \Pi=C\,R\,T \qquad (1.39) \)