## Conceptual Physics (12th Edition)

Assume they are at the same temperature. This means they have the same average translational kinetic energy per molecule. $KE = \frac{1}{2} mv^{2}$. The mass of a $UF_{6}$ molecule containing U-235 is less than that of a uranium hexafluoride molecule containing U-238, so the speed v must be greater to have the same average kinetic energy.