Mole (unit)

Unit systemSI base unit
Unit ofAmount of substance

The mole is the base unit of amount of substance in the International System of Units (SI). Effective 20 May 2019, the mole is defined as the amount of a chemical substance that contains exactly 6.02214076×1023 (Avogadro constant) constitutive particles, e.g., atoms, molecules, ions or electrons.[1]

This definition was adopted in November 2018,[2] revising its old definition based on the number of atoms in 12 grams of carbon-12 (12C) (the isotope of carbon with relative atomic mass 12 Da by definition). The mole is an SI base unit, with the unit symbol mol.

The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2H2 + O2 → 2H2O can be interpreted to mean that 2 mol dihydrogen (H2) and 1 mol dioxygen (O2) react to form 2 mol water (H2O). The mole may also be used to represent the number of atoms, ions, or other entities in a given sample of a substance. The concentration of a solution is commonly expressed by its molarity, defined as the amount of dissolved substance per unit volume of solution, for which the unit typically used is moles per litre (mol/l), commonly abbreviated M.

The term gram-molecule was formerly used for essentially the same concept.[3] The term gram-atom has been used for a related but distinct concept, namely a quantity of a substance that contains an Avogadro's number of atoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgBr2 is 1 gram-molecule of MgBr2 but 3 gram-atoms of MgBr2.[4][5]

Definition and related concepts

Amount of substance is a measure of the quantity of substance proportional to the number of its entities. As of 2011, the mole was defined by International Bureau of Weights and Measures as:

  1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.
  2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Thus, by definition, one mole of pure 12C has a mass of exactly 12 g.

However, on 16 November 2018, after a meeting of scientists from more than 60 countries at the General Conference on Weights and Measures in Versailles, France, organised by the International Bureau of Weights and Measures (BIPM), all SI Units were defined in terms of physical constants. This means that each SI unit, including the mole, will not be defined in terms of any physical objects but rather they will be defined by constants that are, in their nature, exact. Such changes will officially come into effect on 20 May 2019.

  • Following such changes, one mole of any substance will be defined as containing 6.02214076×1023 particles.

For example, one mole of hydrogen atoms will be defined as containing 6.02214076×1023 of hydrogen atoms, which has a mass of 1.008 grams.[6]

The molar mass of a substance is the mass of a sample divided by the amount of substance in that sample. This is a constant for any given substance. Since the unified atomic mass unit (symbol: u or Da) is defined as 1/12 of the mass of the 12C atom, it follows that the molar mass of a substance, measured in grams per mole, is numerically equal to its mean atomic or molecular mass expressed in Da.

One can determine the amount of a known substance, in moles, by dividing the sample's mass by the substance's molar mass.[7] Other methods include the use of the molar volume or the measurement of electric charge.[7]

The mass of one mole of a substance depends not only on its molecular formula, but also on the proportions within the sample of the isotopes of each chemical element present in it. For example, the mass of one mole of calcium-40 is 39.96259098±0.00000022 grams, whereas the mass of one mole of calcium-42 is 41.95861801±0.00000027 grams, and of one mole of calcium with the normal isotopic mix is 40.078±0.004 grams.

Since the definition of the gram is not (as of 2011) mathematically tied to that of the atomic mass unit, the number of molecules per mole NA (the Avogadro constant) must be determined experimentally. The value adopted by CODATA in 2010 is NA = (6.02214129±0.00000027)×1023 mol−1.[8] In 2011 the measurement was refined to (6.02214078±0.00000018)×1023 mol−1.[9]

Mass and volume (properties of matter) are often used to quantify a sample of a substance. However, the volume changes with temperature and pressure. Similarly, due to relativistic effects, the mass of a sample changes with temperature, speed or gravity. This effect is very small at low temperature, speed or gravity, but at high speed like in a particle accelerator or theoretical space craft, the change is significant. The amount of substance remains the same regardless of temperature, pressure, speed or gravity, unless a (chemical or nuclear) reaction changes the number of particles.

Other Languages
العربية: مول
অসমীয়া: ম'ল
asturianu: Mol
azərbaycanca: Mol
Bân-lâm-gú: Mol
беларуская: Моль
беларуская (тарашкевіца)‎: Моль
български: Мол
བོད་ཡིག: མོལ།
bosanski: Mol (jedinica)
brezhoneg: Mol
català: Mol
Чӑвашла: Моль
čeština: Mol (jednotka)
Cymraeg: Môl (uned)
Deutsch: Mol
eesti: Mool
Ελληνικά: Γραμμομόριο
español: Mol
Esperanto: Molo
euskara: Mol
فارسی: مول
français: Mole (unité)
Gaeilge: Mól
galego: Mol
한국어: 몰 (단위)
հայերեն: Մոլ
हिन्दी: मोल (इकाई)
Bahasa Indonesia: Mol
íslenska: Mól
italiano: Mole
עברית: מול
ಕನ್ನಡ: ಮೋಲ್
ქართული: მოლი
қазақша: Моль
Kreyòl ayisyen: Mòl
kurdî: Mol
Кыргызча: Моль
latviešu: Mols
Lëtzebuergesch: Mol
lietuvių: Molis (vienetas)
Ligure: Mole
magyar: Mól
македонски: Мол
मराठी: मोल (एकक)
Bahasa Melayu: Mol
монгол: Моль
Nederlands: Mol (eenheid)
日本語: モル
Nordfriisk: Mol
norsk nynorsk: Mol
occitan: Mòl (unitat)
ਪੰਜਾਬੀ: ਮੋਲ (ਇਕਾਈ)
پنجابی: مول
Piemontèis: Mòle
Plattdüütsch: Mol (Eenheit)
polski: Mol
português: Mol
română: Mol
русиньскый: Мол
русский: Моль
shqip: Moli
සිංහල: මවුලය
Simple English: Mole (unit)
slovenčina: Mol (jednotka SI)
slovenščina: Mol (enota)
Soomaaliga: Mole
српски / srpski: Мол (јединица)
srpskohrvatski / српскохрватски: Mol (jedinica)
suomi: Mooli
svenska: Mol
Tagalog: Mole (yunit)
தமிழ்: மோல்
татарча/tatarça: Моль
ไทย: โมล
Türkçe: Mol (birim)
українська: Моль
Tiếng Việt: Mol
文言: 摩爾
Winaray: Mol
粵語: 摩爾