A drop of water has a volume of approximately 0.05 mL. How many water molecules does it contain? The density of water is 1.0 g/cm³.

Key Concepts

Molar Mass of Water

The molar mass of water (H2O) is approximately 18.02 g/mol. This value is essential for converting grams of water to moles, which allows us to relate the mass of water to the number of molecules it contains. Since one mole of any substance contains Avogadro's number of entities (approximately 6.022 x 10^23), knowing the molar mass is crucial for calculating the total number of water molecules in a given volume.

Density and Volume Relationship

Density is defined as mass per unit volume, and for water, it is approximately 1.0 g/cm³. This means that 1 mL of water has a mass of about 1 gram. Understanding this relationship allows us to convert the volume of water (0.05 mL) into mass, which is necessary for further calculations involving the number of molecules.

Avogadro's Number

Avogadro's number (6.022 x 10^23) is the number of atoms, ions, or molecules in one mole of a substance. This constant is fundamental in chemistry for converting between moles and the number of individual particles. Once we determine the number of moles of water from its mass, we can use Avogadro's number to find the total number of water molecules present in the drop.