The density of $H_{2}O$ is $1g/ml$. What is volume occupied by 1 molecule of water.
$1mol\to 18g=6\times {10}^{+23}$

Molar volume of water = $\frac{molarmass}{density}=\frac{18gm}{1g}$

$\therefore$ volume of one molecule of water = $\frac{18}{6.023\ast {10}^{2}3}=2.988\ast {10}^{23}$