The molecular weight of ${\mathrm{O}}_2\text{ and }{\mathrm{SO}}_2\text{ are }32\text{ and }64$

respectively. If one litre of ${\mathrm{O}}_2\text{ at }{15}^{\circ }\mathrm{C}\text{ and }750\mathrm{mm}$

pressure contains '$\mathrm{N}\text{'}$ molecules, the number of molecules in two litres of 

${\mathrm{SO}}_2$ under the same conditions of temperature 

and pressure will be

At same temperature and pressure, equal volumes have equal number of molecules. If $1 lit$. of oxygen consists $N$ molecules then at same temperature and pressure $1 \mathrm{lit}$ of $S{O}_2$ will consists $N$molecules. So $2 lit$. of $S{O}_2$ will contain $2\mathrm{N}$ molecules.