What is the Boltzmann distribution look like if we have a mix of two gasses with very different molecular weights? Say hydrogen at 2 amu and xenon at 132 amu. Do they have equal average momentum and hence the hydrogen has an average velocity 66 times the velocity of the xenon and an energy .5*m*v^2 that is 66 times higher than the xenon? Or do they have equal average energy? But shouldn't they both have just .5*kT of energy per degree of freedom?