A drop of water has a mass of 0.072 g. How many molecules of water are in the drop?

Step 1. Moles. $\dfrac{0.072\ \text{g}}{18\ \text{g/mol}} = 0.004\ \text{mol}$.

Step 2. Molecules. $0.004 \times 6.022 \times 10^{23} = 2.41 \times 10^{21}$.

Step 3. Atoms. $3 \times 2.41 \times 10^{21} = 7.23 \times 10^{21}$.