Answer
The ratio is 1.004.
Work Step by Step
The rms speed is given by equation 13–9.
$$v_{rms}=\sqrt{3kT/m}$$
Calculate the ratio.
$$\frac{v_{rms,UF6,235}}{ v_{rms,UF6,238}}=\sqrt{\frac{ m_{UF6,238}}{ m_{UF6,235}}}$$
$$=\sqrt{\frac{ 238.050788+6(18.998403)}{ 235.043930+6(18.998403)}}\approx1.004$$
