Force exerted on the wall of a container.

[Dan.kl]

I am doing a question about oxygen in a cubical container, and am being asked to find the average force a molecule exerts on one of the walls of the container. I know the force is equal (in magnitude) to the rate of change of momentum of the molecule, but how can I know the duration of the collision?
The solution calculates the rate of change of momentum by using the time it takes for the molecule to go back and forth between the walls, but this is not the dt involved in F=dp/dt as the dt should be the time that the force acts for, surely?

 

[Steely Dan]

Sure it is. If you are asked the average force, what is implied by any averaging process is to apply the average over a continuous period of time. Assuming that many bounces occur in any chosen time interval, then a good portion of that time is spent exerting no force on the wall, so this must be factored in when computing the average force.