🪴 GoDeep
Search
← University Challenge
Portrait of The mole

Science · Chemistry

The mole

1894 (named); 2019 (redefined)

Chemistry — SI units and stoichiometry

The mole is the SI base unit of amount of substance, symbol mol. It is the chemist's counting unit: the bridge between the grams on a balance and the number of atoms or molecules actually reacting. Since the SI revision that took effect on 20 May 2019 it has been defined as exactly 6.02214076 × 10^23 elementary entities, a fixed number rather than a measured one; before that it was the number of atoms in 12 grams of carbon-12. That number is the Avogadro constant, named in honour of Amedeo Avogadro, who never knew it — it was Jean Perrin who put Avogadro's name to it, a century after Avogadro's hypothesis that equal volumes of gases contain equal numbers of molecules.

How to spot them

  • ▸The SI base unit of amount of substance, symbol mol
  • ▸Defined since 2019 as exactly 6.02214076 × 10^23 entities
  • ▸Formerly the number of atoms in 12 grams of carbon-12
  • ▸The number is named after an Italian who never measured it
  • ▸Celebrated on 23 October between 6.02 a.m. and 6.02 p.m.

Key facts

The SI base unit of amount of substance, symbol mol

One of the seven base units, alongside the metre, kilogram, second, ampere, kelvin and candela.

Since 2019 it has been defined as exactly 6.02214076 × 10^23 elementary entities (2019)

The number is now fixed by definition, so the constant has no uncertainty; the change took effect on 20 May 2019, World Metrology Day.

Until 2019 it was defined as the number of atoms in twelve grams of carbon-12

That definition tied it to a measured mass, so the constant carried an experimental uncertainty.

The number is named after the Italian who proposed in 1811 that equal volumes of gases hold equal numbers of particles (1811)

Amedeo Avogadro died in 1856 without any idea of its magnitude; Jean Perrin attached his name to it around 1909.

Wilhelm Ostwald coined its name in the 1890s, from the German Molekül

German 'Mol' was clipped from Molekulargewicht, molecular weight.

American classrooms celebrate it on 23 October, between 6.02 in the morning and 6.02 in the evening

Written American-style, 6:02 10/23.

Alone among the SI base units, it counts things rather than measuring a physical quantity

Which is why some physicists argue it should never have been a base unit at all.

Key ideas

  • The point of it is proportion. One of these of any substance contains the same number of particles, so a balance reading in grams can be converted directly into a count of reacting particles — which is what a balanced equation actually specifies.
  • The number is unimaginably large: that many grains of rice would bury the British Isles kilometres deep, yet it is the number of water molecules in about 18 grams — roughly a tablespoon.

Quick-fire facts

  • •The Avogadro constant is 6.02214076 × 10^23 per mole, exactly, since 2019.
  • •Jean Perrin, who named the constant after Avogadro, won the 1926 Nobel Prize in Physics for work on the discontinuous structure of matter, including measuring it.
  • •Josef Loschmidt made the first credible estimate of molecular numbers in 1865; German-speaking countries still often call the related figure the Loschmidt constant.
  • •The 2019 redefinition also redefined the kilogram, the ampere and the kelvin in terms of fixed constants.
  • •One of these of an ideal gas occupies 22.4 litres at 0 °C and 1 atmosphere, or 24 litres at room temperature and pressure.
  • •Molar mass in grams per mole is numerically equal to relative atomic or molecular mass — the whole convenience of the unit.
  • •The plural of the symbol is still 'mol', and the unit is never capitalised even though it honours nobody.
  • •Common misconception: it is not a mass and not a volume. It is a count, like a dozen — just a very large one.

Drill it — 9 cards

1 / 9new card

Sources