Introduction:
Vapour density is the ratio of the molar mass of a gas to the molar mass of hydrogen gas under the same conditions of temperature and pressure. It is an important property used to determine the degree of dissociation of a compound.

Relationship between Vapour Density and Degree of Dissociation:
The degree of dissociation of a compound refers to the extent to which it splits into its constituent elements or ions. It can be expressed as the ratio of the number of moles of dissociated molecules to the total number of moles initially present.

1. Equilibrium and Dissociation:
In a chemical equilibrium, the forward and backward reactions occur simultaneously, resulting in a dynamic balance between the reactants and products. The equilibrium constant (K) is a measure of the extent of the reaction at equilibrium.

2. Vapour Density and Molecular Mass:
Vapour density is directly related to the molecular mass of a gas. It is determined by comparing the density of the gas with that of hydrogen gas, which has a known molecular mass of 2 g/mol. The formula for calculating vapour density is: Vapour Density = Molecular Mass of Gas / Molecular Mass of Hydrogen.

3. Effect of Dissociation on Vapour Density:
When a compound dissociates, its molecular mass changes due to the formation of smaller particles. This affects the vapour density of the gas. The degree of dissociation is directly proportional to the change in molecular mass.

4. Example:
Let's consider the dissociation of a compound AB. Initially, there are n moles of AB. After dissociation, x moles of AB dissociate into A and B. The remaining (n - x) moles of AB remain undissociated. The total number of moles at equilibrium is (n - x) + x + x = (n + x) moles.

5. Calculating Vapour Density:
To calculate the vapour density, we need to determine the molecular mass at equilibrium. The molecular mass of the dissociated compound AB is the sum of the masses of A and B. The molecular mass of the undissociated compound AB remains the same.

6. Conclusion:
In summary, the vapour density of a gas is related to the degree of dissociation. As the degree of dissociation increases, the molecular mass of the gas changes, leading to a change in its vapour density. This relationship is important in understanding the equilibrium behavior and composition of gases.