1. The problem statement, all variables and given/known data The mean free path for an ideal collisional gas can be calculated as shown on thishttp://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/menfre.html" [Broken]. I understand the derivation, except for one thing. 2. Relevant equations 3. The attempt at a solution The molecule is at one end of the cylinder at the beginning, and any of the particles could have collided with the molecule at that instant. The same consideration has to go when the molecule reaahes the other end of the cylinder. Does that not mean that we should attach hemispherical volumes to the ends of the cylinder, and take into account any collisions at the end. Please help!!