We separate U- 235 from $\mathrm{U}-238$ by fluorinating a sample of uranium to form UF $_{6}($ which is a gas) and then taking advantage of the different rates of effusion and diffusion for compounds containing the two isotopes. Calculate the ratio of effusion rates for $^{238} \mathrm{UF}_{6}$ and $^{235} \mathrm{UF}_{6}$. The atomic mass of $\mathrm{U}-235$ is 235.054 amu and that of $\mathrm{U}-238$ is 238.051 amu.