The number of water molecules present in a drop of water weighing 0.05 mg is

${\text{18g of H}}_2\text{O}\to 6.023\times {10}^{23}{\text{H}}_{\text{2}}\text{O }\text{molecules}$

Mass of water given $\text{=}\text{0}\text{.05 mg}$

$\text{1}\text{g =1000 mg}$  or $1{\text{mg = 10}}^{-3}\text{g}$

So , $0.05\text{mg}\text{=}\text{0}\text{.05}\times {\text{10}}^{-3}\text{g}$

$0.05\times {10}^{-3}{\text{g of H}}_{\text{2}}\text{O }\to {\text{ ? H}}_2\text{O molecules}$

$\text{Number}\text{of water molecules}=\frac{0.05\times {10}^{-3}\text{g}{\text{H}}_{\text{2}}\text{O}\times 6.023\times {10}^{23}\text{molecules}}{18\text{g}{\text{H}}_{\text{2}}\text{O}}$

                                              $=16.73\times {10}^{17}$ water molecules.