One drop of water has the volume of about 0.05 mL. Because the density of water is 1, the mass is 0.05 g. The molar mass of water (H2O) is 18.0 grams/mol (1.008 + 1.008 + 16.0). This means there is one mole of water in 18.0 grams. One mole is 6.02 × 1023 molecules. Then you can convert grams to number of atoms: