Answer
$1.67\times10^{21}\, molecules$
Work Step by Step
Volume of water= $0.050\, mL=0.050\,cm^{3}$
Mass of water= $\text{Density}\times \text{Volume}$
$=(1.00\, g/cm^{3})(0.050\, cm^{3}) $
$=0.050\, g$
$\text{Number of moles}=\frac{\text{Mass in grams}}{\text{Molar mass}}$
$=\frac{0.050\,g}{18.0\,g/mol}=0.00277778\,mol$
Number of molecules= $(0.00277778\,mol)(6.022\times10^{23}/mol)$
$=1.67\times10^{21}\, molecules$
