$\begin{array}{ll} & \mathrm{v}_{\mathrm{rms}} \propto \frac{1}{\sqrt{\mathrm{M}}} \\ \therefore \quad & \frac{\left(\mathrm{v}_{\mathrm{rms}}\right)_2}{\left(\mathrm{v}_{\mathrm{rms}}\right)_1}=\sqrt{\frac{\mathrm{M}_1}{\mathrm{M}_2}} \\ & \text { Given that }\left(\mathrm{v}_{\mathrm{rms}}\right)_2=4\left(\mathrm{v}_{\mathrm{rms}}\right)_1 \\ \therefore \quad & \sqrt{\frac{32}{\mathrm{M}_2}}=4 \\ \therefore \quad & \frac{32}{\mathrm{M}_2}=16 \\ \therefore \quad & \mathrm{M}_2=2\end{array}$