Question

what are the assumption that are made when we write that the change of momentum on collision with the wall is 2mu when molecules of mass m moving in the the interior with velocity u collide with wall??​

Answer 1

The assumption are that

1. the collision are elastic (otherwise a molecule colliding with the momentum mu need not rebound with the momentum -mu so the change is $mu-(-mu)=2mu$
2. there are no force of attraction and repulsion between the gas molecule (as otherwise, a molecule moving with a velocity u would not reach the wall with the velocity u). Of course, there is one more assumption: the wall is fixed.