The gas which is twice as dense as oxygen under the same conditions is

The ratio of density between sulphur dioxide to oxygen is

$\frac{\mathrm{V}.\mathrm{D} \mathrm{of}\hspace{0.17em}{\mathrm{SO}}_2}{\mathrm{V}.\mathrm{D} \mathrm{of} {\mathrm{O}}_2}=\frac{M.W of\hspace{0.17em}{\mathrm{SO}}_2/\mathrm{M}.\mathrm{W} \mathrm{of}\hspace{0.17em}{\mathrm{H}}_2}{M.W OF O_2/M.W of H_2}=\frac{64/2}{32/2}=2:1$