Under same conditions of temperature and pressure if you collect $3$ litre of ${\mathrm{O}}_2$, $5$ litre of ${\mathrm{Cl}}_2$ and $6$ litre of ${\mathrm{H}}_2$, 

Which has the highest number of molecules?

Under same conditions of temperature and pressure if you collect $3$ litre of ${\mathrm{O}}_2$, $5$ litre of ${\mathrm{Cl}}_2$ and $6$ litre of ${\mathrm{H}}_2$, Which has the lowest number of molecules? 

Which of the following is a triatomic molecule?

The volume occupied by $1$ mole of a diatomic gas at S.T.P is