Which quantity is expressed in moles?

The quantity that is expressed in moles is the amount of substance. In chemistry, the mole is a fundamental unit that quantifies the amount of a substance based on the number of particles it contains, be they atoms, molecules, ions, or other entities. One mole of a substance contains approximately (6.022 \times 10^{23}) particles, known as Avogadro's number.

The mole is essential for relating the mass of substances to the number of particles, facilitating calculations in chemical reactions and stoichiometries. For example, when calculating how many grams of a substance are needed to react with a certain number of moles of another substance, understanding the mole concept is crucial.

Other quantities mentioned, such as mass, electric current, and temperature, each have their own distinct units and measurement systems. Mass is typically measured in kilograms or grams, electric current in amperes, and temperature in degrees Celsius or Kelvin, which are separate from the mole concept that focuses specifically on the amount of substance.