The molecule of water is written as ${\mathrm{H}}_2\mathrm{O}$.

Thus, $$\begin{gathered} \mathrm{Mass} \mathrm{of} \mathrm{molecule} \mathrm{of} \mathrm{water} = (2\times \mathrm{mass} \mathrm{of} \mathrm{hydrogen} \mathrm{atom}) + \mathrm{mass} \mathrm{of} \mathrm{oxygen} \mathrm{atom} \\ \mathrm{Mass} \mathrm{of} \mathrm{molecule} \mathrm{of} \mathrm{water} = (2\times 1.0 \mathrm{g}/\mathrm{mol}) + 16.0 \mathrm{g}/\mathrm{mol} = 18.0 \mathrm{g}/\mathrm{mol}. \end{gathered}$$

Hence, the mass of a molecule of water is 18.0 g/mol.