If one considers an ideal gas, what does the mean time between collissions of the molecules depend upon? molecular size? number per unit volume? temperature of the gas? I'm thinking so far, it must def depend on number density. It would usually depend on molecular size (but I thought an ideal gas was supposed to assume point like particles?). It may not depend on temperature since n=PV/RT, so if you specifically number density then perhaps you already factored in temperature dependence? Anyone know any specific formula for this mean time of flight or how to derive?