Comprehension:
Solutions
The solution which obeys Raoult's law over the entire range of concentration are known as ideal solutions and which does not obey are known as non-ideal solution. The vapour pressure of a non ideal solution is either higher or lower than that predicted by Raoult's law equation. The colligative properties, which depend on the number of solute particles irrespective of their nature, are relative lowering of vapour pressure, depression of freezing point, elevation of boiling point and osmotic pressure. All these properties can be utilised for the determination of molar mass of the solute.
Which of the following is the widely used method to determine the molar mass of macromolecules?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Osmotic pressure
Osmotic pressure is the method widely used for macromolecules:
\( \displaystyle \pi=CRT=\frac{n_2}{V}RT,\quad M_2=\frac{w_2\,R\,T}{\pi\,V} \)
Why the other options are incorrect
Option (1) Elevation of boiling point — Incorrect. The rise is far too small to measure accurately, and heating to the boiling point would denature biomolecules.
Option (3) Depression in freezing point — Incorrect. The fall in freezing point is also too small to measure accurately for such dilute solutions.
Option (4) Relative lowering of vapour pressure — Incorrect. The change in vapour pressure is extremely small and difficult to measure precisely.
NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – …
