Mole
SI base unit · quantity: amount of substance
01 · Definition
The mole is the only SI unit that measures nothing at all. It counts: so many pieces, exactly like a dozen or a hundred, only the number is unimaginably larger.
One mole contains exactly 6.022 140 76·10²³ elementary entities — atoms, molecules, ions, electrons; which ones exactly must always be stated. Since 2019 this number is fixed by definition, and the mole no longer depends on any substance or on the kilogram.
«A mole of oxygen» is a phrase without meaning: a mole of O atoms weighs 16 g, a mole of O₂ molecules 32 g. The SI requires naming the entity: mol(O), mol(O₂), mol(e⁻).
02 · Conversion
Enter an amount — the passport will convert it
The mole has decimal prefixes like everything else, but units of a different logic live alongside it: the pound-mole of American petrochemistry and the normal cubic metre of the gas industry.
Normal conditions are an agreement, and there are two of them: 0 °C and 101.325 kPa give 22.414 l/mol, while «standard» 25 °C already gives 24.466 l/mol. Always check which one a table means.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| nmol | nanomole | 1·10⁹ | biochemistry, trace amounts |
| µmol | micromole | 1·10⁶ | pharmacology, vitamins, photosynthesis |
| mmol | millimole | 1000 | blood tests, laboratory chemistry |
| mol | mole | 1 | the base notation of amount in SI |
| kmol | kilomole | 0.001 | industrial reactors, gas balances |
N = n · N(A) with N(A) = 6.022 140 76·10²³ mol⁻¹; the volume is for an ideal gas at 0 °C and 101.325 kPa
03 · Orders of magnitude
from a single molecule to a planet's atmosphereA mole: 18 g of water, 44 g of carbon dioxide
04 · Measuring instruments
What moles are measured out with
Moles are never weighed directly: one weighs grams and divides by the molar mass. A resolution of 0.1 mg is the workhorse of chemistry.
It accelerates ions and sorts them by mass: it gives the isotopic composition, and hence the exact molar mass of a sample.
It counts moles from the charge that has passed, via the Faraday constant: electrochemistry turns amperes straight into amount of substance.
The XRCD method: in a perfect sphere of silicon-28 the atoms are literally counted through the lattice parameter — that is how the Avogadro number was measured before it was fixed.
05 · Writing rules
All lowercase: mol, and always say a mole of what
The symbol mol is written in lowercase, without a full stop, because the name is not a surname. In English the plural is «moles», but the symbol never takes an s: 5 mol, not 5 mols. And «0.1 M» is molarity, mol/l — a concentration, not an amount.
06 · Neighbouring units
The mole joined the SI last of all — in 1971 — and brought one derived unit with it, the katal: moles per second, the rate of an enzymatic reaction. Beside it stand concentration in mol/m³ and molar mass in g/mol, which tie chemistry to the kilogram.
Three formulas that make up the whole of practical chemistry: entities, gases and electrolysis.
07 · Historical section
How substance was counted before the mole
A mass of substance in grams, numerically equal to the molecular mass. The same amount as a mole, but expressed through the balance.
The same for atoms rather than molecules: 16 g of oxygen is a gram-atom, 32 g a gram-molecule. Precisely the confusion the mole removed.
The amount that reacts with one atom of hydrogen. It depended on the reaction rather than on the substance — which is why it was displaced.
The same principle but with the pound instead of the gram: it lives on in American petrochemistry and in distillation-column calculations.
In 2019 the mole stopped depending on the kilogram
The former definition through twelve grams of carbon-12 tied chemistry to the mass standard: as long as the kilogram was a lump of platinum, the Avogadro number was a measured quantity with an uncertainty. Fixing the number turned the relation around — now the molar mass of carbon-12 is what carries the uncertainty, and it is tiny: 12 g/mol to within 3·10⁻¹⁰. For any practical chemistry the old and the new mole are indistinguishable.
Catalogue · The International System of Units
A passport for every quantity
Seven base units, twenty-two derived units with names of their own, and an archive of systems withdrawn from use. Each is a sheet of its own: definition, dimension, conversion, instruments, writing rules. In 36 languages.
Base units
open the mole's passportDerived units with names of their own
in terracotta — those whose definition contains the moleHistorical systems
archive of notationsPassport language
36 languages. The symbol mol and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.