Osmotic pressure measurements are less accurate and difficult to carry out. Therefore, this method is not useful for measuring molecular weights. But it is used to find out the molecular weights of polymers, e.g., plastics, proteins, and starch having many monomer units. Knowing the value of the osmotic pressure of a given solution, the molecular weight of the solute can be calculated using van't Hoff equation.
When osmotic pressure and temperature for two solutions are same, equal volumes of solution would contain same number of:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Moles of solute
From the van’t Hoff equation
\( \displaystyle \pi = \frac{n_2}{V}RT \quad\Rightarrow\quad n_2 = \frac{\pi V}{RT} \)
If two solutions have the same osmotic pressure π and the same temperature T, then for equal volumes V they contain the same number of moles of solute n2. Such solutions are isotonic, and they have equal molar concentrations.
Why the other options are incorrect
Option (2) Moles of solvent — Incorrect. Osmotic pressure depends on the solute particles; the amount of solvent does not appear in the equation.
Option (3) Moles of solution — Incorrect. “moles of solution” is not a quantity in the equation; only the solute is counted.
Option (4) Molecules of solutions — Incorrect. The total number of molecules includes the solvent and need not be equal.
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:
\( \Pi = C\,R\,T \) (1.39)
Here Π is the osmotic pressure and R is the gas constant.
\( \Pi = (n_2/V)\,R\,T \) (1.40)
Here V is volume of a solution in litres containing n2 moles of solute. … Two solutions having same osmotic pressure at a given temperature are called isotonic solutions.”