Question

Which of the following is ‘NOT’ a colligative property?
A) Elevation in boiling point
B) Depression in freezing point
C) Osmotic pressure
D) Lowering of vapour pressure

Answer 1

Hint: Recall the four colligative properties exhibited by a solution. One of property states that relative lowering in vapour pressure of a dilute solution is equal to the mole fraction of the solute in the solution. The vapour pressure of the solution decreases when a non-volatile solute is added to a volatile solvent.

Complete step by step answer:
All those properties which depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution are called colligative properties. Colligative properties are mostly seen in dilute solutions. These properties depend only on the number of solute particles and not on the type or identity of solute present. There are four types of colligative properties exhibited by a solution. These are:
- Relative lowering of vapour pressure
- Elevation in boiling point
- Depression in freezing point
- Osmotic pressure

In the given options, A, B and C are among the colligative properties but option D, which says lowering in vapour pressure, is incorrect. Relative lowering in vapour pressure is the colligative property but not the lowering in vapour pressure.
So, the correct answer is “Option D”.

Note: Raoult established a relation between relative lowering in vapour pressure and mole fraction of the solute. The relation given by Raoult is:
$\dfrac{{p_1^0 - {p_1}}}{{p_1^0}} = \dfrac{{{n_2}}}{{{n_1} + {n_2}}}$
where, $p_1^0$ is the vapour pressure of pure solvent, ${p_1}$ is the vapour pressure of the solvent in solution, ${n_1}$ and ${n_2}$ are number of moles of solvent and solute respectively.
The left hand side of the equation is called relative lowering in vapour pressure is equal to the mole fraction of the solute.