The diameter of the water vapour molecule is $2\times {10}^{-10} \mathrm{m}$ and the number of molecules per unit volume at STP is $2.7\times {10}^{25} {\mathrm{m}}^{-3}.$ The mean free path for a water molecule in water vapour at \(373\) \(\mathrm{K}\) is:
1.  $2.5\times {10}^{-7} \mathrm{m}$
2.  $6\times {10}^{-7} \mathrm{m}$
3.  $3\times {10}^{-7} \mathrm{m}$
4.  $4\times {10}^{-7} \mathrm{m}$