A-level Chemistry/OCR (Salters)/Amount of substance

Amount of substance is a physical quantity. Its symbol is n and its SI unit is the mole, whose symbol is mol.

One mole of objects is 6.0221415 × 1023 objects. That's 60 221 415 000 000 000 000 000, or about sixty billion trillion objects. The number 6.0221415 × 1023 is called Avogadro's number or Avogadro's constant and has the symbol L. This number is usually rounded down to 6.02 × 1023 or 6 × 1023.

One mole of a molecular substance is 6 × 1023 molecules. One mole of a non-molecular substance (such as NaCl) is 6 × 1023 formula units, e.g. 6 × 1023 lots of NaCl, so 6 × 1023 sodium ions and 6 × 1023 chloride ions.