In the derivation of finding pressure exerted by a gas using kinetic theory of gases I am not understanding why the time between two collisions is taken as the time for rate of change of momentum when a particle bounces back from the wall. please help me

Dear student,

This is not true. We do not take time between two collisions to be the time between two collisions. It must have been some incorrect derivation that you have seen. Infact, the time is taken to be unknown, but anyways after we consider the root mean square of each particle along the direction perpendicular to the wall and number density of the gas, the time term itself gets eliminated.