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The mole, Avogadro's number and formula mass

Summary

A mole is a counting unit for particles, like a dozen but far larger. It holds about 6.022 × 10^23 entities, a number called Avogadro's number. The mass of one mole of a substance in grams equals its average formula mass in atomic mass units, which links weighing something to counting its atoms. Formula mass is the sum of the average atomic masses of all the atoms in the formula.

Draft. License: CC BY 4.0 (this summary). Sources: [S1] OpenStax (Rice University), "Chemistry, 3.1 Formula Mass and the Mole Concept" (CC BY 4.0); [S2] National Institute of Standards and Technology (NIST), "SI base unit: mole" (public domain (US government work)).

Facts

• id: F1 claim: Formula mass is the sum of the average atomic masses of all atoms in a substance's formula; for covalent compounds it is also called molecular mass. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F2 claim: Examples from the text: chloroform (CHCl3) 119.37 amu; aspirin (C9H8O4) 180.15 amu; sodium chloride (NaCl) 58.44 amu. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F3 claim: For ionic compounds, 'molecular mass' is not strictly correct because the formula does not describe a discrete molecule; neutral-atom masses are used, since electron mass is negligible. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F4 claim: The textbook defines a mole as the number of entities equal to the atoms in exactly 12 g of carbon-12, about 6.022 × 10^23 per mole (Avogadro's number). confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F5 claim: Since the 2019 SI revision, the mole is defined by fixing Avogadro's constant at exactly 6.02214076 × 10^23 per mole, rather than by carbon-12. confidence: high date: 2026-11-02 sources: [S2] license: CC BY 4.0 checked_on: 2026-11-02
• id: F6 claim: One mole of any element contains the same number of atoms as one mole of any other element, but their masses differ; the molar mass in grams per mole numerically equals the average atomic or formula mass in amu. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02

Caveats

• The textbook (first edition) uses the pre-2019 carbon-12 definition; the numbers barely differ for everyday calculations.

Pointers

• id: P1 answers: Current SI definition of the mole where: https://www.nist.gov/si-redefinition/mole access: free, public web page

@misc{iil_note_xn1q46m6,
  author = {{iinventedlight Library}},
  title = {{The mole, Avogadro's number and formula mass}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-02},
  url = {https://iinventedlight.com/library/reference/us-gov/mole-and-avogadro/},
  note = {Permanent ID iil:note:xn1q46m6. License: CC-BY-4.0. Summary of https://openstax.org/books/chemistry/pages/3-1-formula-mass-and-the-mole-concept.},
}

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