The number of molecules in one litre of water is (density of water = 1g/mL)

$$\begin{gathered} 1 \mathrm{lit} \to 1000\mathrm{gms} \\ \mathrm{No}.\mathrm{ofmoles} =\frac{1000}{18} \mathrm{moles} \mathrm{No}.\mathrm{molecules} \frac{1000}{18}\times 6\times {10}^{23} \\ =3.33\times {10}^{25} \mathrm{molecules} \\ 1.5\times {10}^{20}\to 15\mathrm{mg} \end{gathered}$$