From the relation of rms velocity, we know $ {{c}_{rms}}\propto \sqrt{\frac{1}{M}} $ Hence, $ \frac{{{c}_{rms(H)}}}{{{c}_{rms(H)}}}=\sqrt{\frac{1/{{M}_{H}}}{1/{{M}_{O}}}}=\sqrt{\frac{{{M}_{O}}}{{{M}_{H}}}} $ $ =\sqrt{\frac{32}{2}}=\sqrt{16}=4 $ $ {{c}_{rms}}=\frac{{{c}_{rms(H)}}}{4}=\frac{1}{4}\times 1.8\,km/s $