The volume of a drop of water is $0.05\text{mL}$ and the density of water is $1.0\text{g/mL}$. How many water molecules are present in a drop of water?

The correct option is A $1.7\times {10}^{21}$
As, $\text{Density}=\frac{\text{mass}}{\text{volume}}$
So, $\text{mass}=1.0\text{g/ml}\times 0.05\text{mL}=0.05\text{g}$
Using Avogadro's law, $18\text{g}$ of $H_{2}O$ contains $6.023\times {10}^{23}$ molecules.
So, $0.05\text{g}$ of $H_{2}O$ contains $=\frac{6.023\times {10}^{23}}{18}\times 0.05=1.67\times {10}^{21}$ molecules.