Answer
$2\times10^{21}$molecules of water
Work Step by Step
Multiply the volume of 0.05mL by the given density of water (remember that $cm^{3}$ is the same as milliliters). Then multiply by the molar mass of water which can be calculated by adding the molar mass of its constituent elements. Finally, multiply by Avogadro's number to get the number of molecules. Notice how all units but molecules cancel each other out.
$0.05 mL \times \frac{1g}{1mL}\times\frac{1 mol}{18.02g}\times\frac{6.022\times10^{23}molecules}{1 mol} = 2\times10^{21} molecules$
