Mole

Amount of substance requires a specified entity, such as an atom, molecule, ion or defined group of particles. A mole of hydrogen atoms and a mole of hydrogen molecules refer to different objects. Chemical formulas therefore matter when converting between molecules and their constituent atoms.

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The current definition fixes the numerical value of the Avogadro constant, making the number of entities per mole exact. Adopted in 2018 and effective on 20 May 2019, it replaced the definition based on the number of atoms in a particular mass of carbon-12.

References: [1] [2]

Chemists commonly infer amount from measured mass and molar mass rather than count particles individually. Molar mass depends on chemical composition and isotopic abundances. An exact defining constant does not eliminate uncertainties arising from balances, material purity or the preparation of a real sample.

References: [3] [1]

Balanced chemical equations express entity ratios and corresponding mole ratios, linking microscopic reactions with laboratory quantities. Equal amounts of different substances contain equal specified entity counts, but their masses and volumes generally differ. Volume additionally depends on temperature, pressure and physical state.

References: [3]