For a fixed mass of a gas at constant temperature the pressure falls when the volume increases, and vice versa. Explain according to the kinetic theory?

Explanation

When volume increases at constant temperature the intermolecular distance SO there are less number of molecules is unit volume. As a result the number of collisions of the molecules per second with the unit area of wall of the container decreases. Thus the change of momentum of the molecules i.e., force exerted by the molecules decreases.. This is why the pressure of the gas falls.
Conversely the pressure rises due to increased number of collisions per second when the volume of a gas decreases at constant temperature.