Answer
1.4
Work Step by Step
Here we use the equation $KE_{Avg}=\frac{1}{2}mv_{rms}^{2}$ to find the ratio of the diffusion rates for the two types of atoms at a fixed temperature.
$KE_{Avg}=\frac{1}{2}mv_{rms}^{2}$
$v_{rms}=\sqrt {\frac{2KE_{Avg}}{m}}$
We also know that,
$KE_{Avg}=\frac{3}{2}kT$ Therefore, for a fixed temperature average kinetic energy is equal.
The ratio of the = $\frac{R_{A}}{R_{B}}=\frac{v_{rms,A}}{v_{rms,B}}=\frac{\sqrt {\frac{2KE_{Avg}}{m_{A}}}}{\sqrt {\frac{2KE_{Avg}}{m_{B}}}}=\sqrt {\frac{m_{B}}{m_{A}}}$
diffusion rates
Let's plug known values into this equation.
$$\frac{R_{A}}{R_{B}}=\sqrt {\frac{2\space u}{1\space u}}=1.4$$