The mass of a molecule of water is

$\begin{array}{l}6\times {10}^{23}\mathrm{molecules}\mathrm{has}\mathrm{mass}=18\mathrm{g}\\ 1\mathrm{molecules}\mathrm{has}\mathrm{mass}=\frac{18}{6\times {10}^{23}}\\ =3\times {10}^{-23}\mathrm{g}\\ =3\times {10}^{-26}\mathrm{kg}\end{array}$