Question

Which of the following is not a colligative property?
A- Osmotic pressure
B- Optical activity
C- Depression in Freezing point
D- Elevation in Boiling point

Answer 1

Hint: Try to figure out the meaning of colligative property. Properties that are being affected by the concentration of solute dissolved in the solution are called colligative properties. Verify options whether it is a colligative property or not.

Complete step by step answer: We know that Colligative properties are those which depend on the concentration of solute present in a solution. These properties depend on the composition of the solution. We will eliminate the options and decide which is not a colligative property.
Osmotic pressure is the pressure difference between pure solvent and solution with the non-volatile solute. This pressure is dependent on the concentration of solute.
Osmotic pressure P = $\dfrac { nRTi }{ V } \quad =\quad cRTi$
Here c is concentration, R is the universal gas constant, T is temperature, i is Vant Hoff factor.
Therefore osmotic pressure is dependent on concentration so it is a colligative property.
Optical activity depends on the solvent and does not depend on the solute so it is not a colligative property.
Depression in freezing point is the lowering of the freezing point of a solvent when a non-volatile solute is added to it.
\[\triangle { T }_{ f }\quad =\quad i{ k }_{ f }m\]
$\triangle { T }_{ f }$ is the decrease in freezing point, i is Vant Hoff factor, $\ { k }_{ f }$is cryoscopic constant and m is the molality of nonvolatile solute. It is a colligative property.
Elevation in the boiling point is an increase in the boiling point of a solvent when a nonvolatile solute is added to it.
\[\triangle { T }_{ b }\quad =\quad i{ k }_{ b }m\]
$\triangle { T }_{ b }$ is the decrease in freezing point, i is Vant Hoff factor, $\ { k }_{ b }$ is ebullioscopic constant and m is the molality of nonvolatile solute. It is a colligative property.

Therefore option B is correct.

Note: We may think that Depression in freezing point and elevation in boiling point is dependent on molality, not concentration but molality increases with an increase in concentration. Molality and concentration both depend on the number moles of solute.