In kinetic theory of gases, a molecules of mass ‘m’ of an ideal gas, collide with a wall of vessel with velocity ‘v’. The change in the linear momentum of the molecule is

Δp→=pfinal−p¯lnitial  ;Δp→=change in momentum;   pfinal=final momentum;p¯lnitial=initial momentum;m=mass;v=velocity ΔP→=P¯f−P¯l=m(−v)−mv=−2mv if initial velocity=v; fina velocity =-v; negative  since opposite in direction to v |ΔP|=2mv