Glossary
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.
The molar mass of a substance is the mass of one mole of it, in grams per mole. It is read off the periodic table rather than looked up: add the atomic masses of every atom the formula contains. Water is two hydrogens and one oxygen, 2 × 1.01 + 16.00, so 18.02 g/mol; table salt is 22.99 + 35.45 = 58.44 g/mol. A unit conversion is the whole idea. One mole is 6.022 × 10²³ particles, and one mole of a substance in grams weighs its formula mass, so grams become moles by dividing and moles become grams by multiplying. moles = mass ÷ molar mass, and mass = moles × molar mass.
The one step people drop is a subscript outside a parenthesis. In Ca(OH)₂ the 2 multiplies everything inside the bracket, so the formula holds one calcium, two oxygens and two hydrogens: 40.08 + 32.00 + 2.02 = 74.10 g/mol. Hydrated salts and formulas with several groups behave the same way. Carry the unit with the number — g/mol — and a mass that has been divided by nothing looks wrong before you check it.
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Related terms
- 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.
- 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.
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)