Number of molecules in one litre of oxygen at STP conditions is

One mole of any substance has $6.022\times {10}^{23} \mathrm{molecules} \mathrm{or} \mathrm{atoms}$ and volume occupied for one mole is 22.4 L. Number of molecules present in one litre of oxygen at STP conditions is:

$\frac{6.022\times {10}^{23}}{22.4}$