Glossary
mole
A mole is 6.022 × 10²³ particles of a substance — the chemist's counting unit, chosen so that one mole of a compound weighs its formula mass in grams: 18 g of water is one mole of it.
A mole is a count, in the same way a dozen is a count: 6.022 × 10²³ of whatever particles are being counted. The number is fixed so that one mole of a substance has a mass in grams equal to its formula mass, which is what makes the mole useful — it converts between the number of particles, which no instrument measures directly, and the mass, which a balance reads. To find the mass of one mole of a compound, add the atomic masses of every atom in its formula; water is 18 g/mol, sodium chloride is 58.5 g/mol. A question can ask any of three things: the mass of a given number of moles, the number of moles in a given mass, or the number of particles in either. All three are the same relation rearranged. What it is not is a molecule count — one mole of water molecules contains two moles of hydrogen atoms.
Taught in
Related terms
- molar mass — Molar mass is the mass of one mole of a substance, in grams per mole, read off the periodic table by adding the atomic masses in the formula.
- molarity — Molarity is concentration in moles of solute per liter of solution — not per liter of solvent — and it falls when a solution is diluted, because the moles of solute stay and the volume grows.
- atomic number — An element's atomic number is the number of protons in its nucleus, and it is what decides which element the atom is.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)