What does the mole measure?

• A. Mass
• B. Amount of substance
• C. Volume
• D. Temperature

Answer: (B)

The mole is a counting unit for chemical particles, telling you how much substance you have by the number of particles it contains. One mole corresponds to 6.02 × 10^23 particles (Avogadro’s number), whether they are atoms, molecules, or ions.

This lets us connect the amount of substance to its mass using the molar mass. If you know the mass and the molar mass, you can find the amount in moles with n = mass / molar mass. For example, water has a molar mass of about 18 g/mol, so 18 g of water is about 1 mole, containing roughly 6.02 × 10^23 water molecules.

Volume and temperature describe how much space the substance takes or how hot or cold it is, not how many particles are present. For gases at the same temperature and pressure, the amount in moles relates to volume, but the mole itself is defined as a count of particles, not as a measure of mass, volume, or temperature.