The molecular weight of ${\mathrm{O}}_2$ and ${\mathrm{SO}}_2$ are $32 \mathrm{g} {\mathrm{mol}}^{-1}$ and $64 \mathrm{g} {\mathrm{mol}}^{-1}$ respectively. If one litre of ${\mathrm{O}}_2$ at $15 °\mathrm{C}$ and $750 \mathrm{mm} \mathrm{Hg}$ pressure contains $\mathrm{N}$ molecules, the number of molecules in two litres of ${\mathrm{SO}}_2$ under the same conditions of temperature and pressure will be:

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