How can separate 1L samples of O2 and H2 have the same average velocity/

Answer 1

The kinetic energy of gaseous molecules is proportional to the temperature. At the same temperature, the kinetic energy is the same for any gas.
K.E. = 1/2 mv². Therefore, at the same T, the average velocity of the lighter molecules will be higher, the heavier molecules will be slower. Having O2 and H2 at different temperatures will allow the average velocity to be the same