calculate the volume occupied by one molecule of water?

Density of water is approximately 1g/ml
Thus 1g of water occupy volume  = 1 ml
Molar mass of water = 18 g/mol
​Number of moles of water in 1g = $\frac{1}{18}$ moles = 0.055 moles H₂O

​Now 18 g water contain = 6.022$\times$10²³ molecules H₂O
Thus 1 g water will have = $\frac{6.022\times {10}^{23}}{18}$ molecules H₂O = 0.334 ​$\times$10²³ molecules H₂O

This shows that 
 0.334 $\times$10²³ molecules H₂O  will occupy volume = 1 ml
Therefore volume occupied by 1 H₂O molecule = $\frac{1}{0.334\times {10}^{-23}}$ = 2.99 ​$\times$10^-23 ml

Thus volume occupied by 1 H₂O mlecule is ​2.99 ​$\times$10^-23 ml.