Volume occupied by one molecule of water (density = $1 \mathrm{g} {\mathrm{cm}}^{-1}) \mathrm{is}$

m = V x d 

18 = V x 1 = V = 18 m/

$6.02\times {10}^{23}$ molecules occupied $=18{\mathrm{cm}}^3$

1 molecule occupied $=\frac{18}{6.02\times {10}^{23}}=3\times {10}^{-23}{\mathrm{cm}}^3$