From gas kinetic theory, one has the result that the total internal energy of a mole of ideal gas particles is equal to the sum of the kinetic energies of each of the particles. For one mole of gas:
U = (3/2)*R*T
where R is the universal gas constant (8/314 J/(mol*K) and T is the thermodynamic temperature.
At 320K, a the total kinetic energy of the molecules in a mole of Kr is:
U = (3/2)*(8.314 J/mol*K)*(320K) = 2660 J
One mole of Kr gas has a mass of 83.80 gm, so the center-of-mass velocity of a mole of Kr that has a translational kinetic energy of 2660 J would be:
2660 J = (1/2)*(8.38*10^-2 kg)*v^2
6.35*10^4 (m/s)^2 = v^2
252 m/s = v
