Answer
On average, U-238 atoms move slower than U-235 atoms in a mixture. The slower, heavier gas will diffuse more slowly through a membrane.
Work Step by Step
Being in a mixture means that gas molecules 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 U-238 is greater than that of U-235, so the speed v must be less to have the same average kinetic energy.
