1. The problem statement, all variables and given/known data The equation for the "average" velocity of a gas molecule in a closed container is: Vrms = ( 3 x k x T / m )^ 1/2 where k = 1.38 x 10 ^ 23 Joules/K T is temp in Kelvin m = mass per ion in kg = Atomic Mass M / Avagadro's Number Na so for N2, I can calculate m = (28 grams/mole) / 6.02 x 10^23 ions/mole and get m = 4.65 x 10^-23 grams/ ION , which I change to 4.65 x 10^-26 kg/ION 2. Relevant equations Vrms = ( 3kT/m)^1/2 I read the 28 grams/mole from the Atomic Table. ( 14 + 14 for N2 ). And I realize that m must be in kg because of the units of k. 3. The attempt at a solution ( trying to edit out some errors here ) Now, I'm given the problem of calculating Vrms of a neutron in the core of a reactor at 400K. I looked up the mass of the neutron on-line and got 1.675 x 10^-27 kg .. OK, fine! But I can't use that for Atomic Mass M. Instead the proper M = 1.0 x 10^-3 kg/mole And the lab book uses the formual Vrms = ( 3k Na T )/ M )^1/2 where m = M / Na .. atomic weight / Avagadro's number This must be an elementary question, but why is M = 10^-3 kg/mole for the neutron ???? .. as used in the book ??? What is a mole of neutrons ??