Question

Which of the following is not a colligative property?

(A) Elevation in boiling point

(B) Depression in freezing point

(C) Osmotic pressure

(D) Refractive index

Answer 1

Colligative properties are those properties which are dependent on the number of particles present in the solution. So, to answer this question we will look for those properties which depend on the concentration that is molarity or molality of the solute.

Complete step by step solution:

Let’s look at the answer of the given question:

First we will define elevation in boiling point.

When a non-volatile solute is added to a solvent then the boiling point of the solvent is increased. This is known as elevation in boiling point. Elevation in boiling point is a colligative property as the ratio of solute-solvent particles.

The formula for elevation in boiling point is:

$\Delta {{T}_{b}}\,=\,{{K}_{b}}\times \,m$

Where, $\Delta {{T}_{b}}\,$is elevation in boiling point, ${{K}_{b}}$ is ebullioscopic constant and ‘m’ is molality of solute.

Now, we will define depression as a freezing point.

When a solute is added to a solvent then the freezing point of the solvent decreases. This is known as depression in freezing point. It is also a colligative property. It depends on the amount of solute added and not on the type of solute added.

The formula for depression in freezing point is:

$\Delta {{T}_{f}}\,=\,{{K}_{f}}\,\times \,m$

Where, $\Delta {{T}_{f}}$ is the depression in freezing point, ${{K}_{f}}$ is the cryoscopic constant and ‘m’ is the molality of the solute.

Now we will define osmotic pressure.

It is the sufficient amount of pressure which is to be applied to prevent the movement of solvent particles. It is a colligative property as it is proportional to the molarity of the solution.

The formula for osmotic pressure is:

$\pi \,=\,CRT$

Where, $\pi$ is the osmotic pressure, ‘C’ is the molarity of the solution, ‘R’ is the gas constant and ‘T’ is the temperature.

Now, we will define a Refractive index.

It is the ratio of speed of light in different mediums. It is not a colligative property as it not dependent on the number of particles present in the medium.

So, we have seen that the Refractive index is the only property which is not dependent on the number of the particles.

Hence, the answer to the given question is option (D)

Note: If the solute is ionic then Van't Hoff factor ‘i’ is used while calculating the colligative properties. ‘i’ is the ratio of the number of ions after dissociation to the number of ions before dissociation.

$i=\dfrac{\text{Number of ions after dissociation}}{\text{Number of ions before dissociation}}$