The amount of substance at the molecular level is quantified by which unit?

• A. Meter
• B. Kelvin
• C. Candela
• D. Mole

Answer: (D)

The amount of substance at the molecular level is quantified by the mole. The mole is an SI base unit used to count entities—such as atoms, molecules, or ions—by amount rather than by their mass or size. One mole corresponds to Avogadro’s number, 6.022 × 10^23 entities. This lets chemists relate a measurable mass to a count of particles: the mass of a mole of a substance (its molar mass) in grams equals the number of moles times the molar mass. For example, one mole of water contains 6.022 × 10^23 water molecules, and its mass is approximately 18.015 grams per mole.

Other common units don’t measure amount of substance: meters measure length, kelvin measures temperature, and candela measures luminous intensity. The mole uniquely counts microscopic entities, providing a bridge between the microscopic world and measurable quantities.