Mole (unit)
mole | |
---|---|
SI | |
Unit of | amount of substance |
Symbol | mol |
The mole (symbol mol) is a
The mole is widely used in chemistry as a convenient way to express amounts of
Concepts
Relation to the Avogadro constant
The
Nature of the entities
Depending on the nature of the substance, an elementary entity may be an atom, a molecule, an ion, an ion pair, or a subatomic particle such as a proton. For example, 10 moles of water (a chemical compound) and 10 moles of mercury (a chemical element) contain equal numbers of substance, with one atom of mercury for each molecule of water, despite the two quantities having different volumes and different masses.
The mole corresponds to a given count of entities.[5] Usually the entities counted are chemically identical and individually distinct. For example, a solution may contain a certain number of dissolved molecules that are more or less independent of each other. However, in a solid the constituent entities are fixed and bound in a lattice arrangement, yet they may be separable without losing their chemical identity. Thus the solid is composed of a certain number of moles of such entities. In yet other cases, such as diamond, where the entire crystal is essentially a single molecule, the mole is still used to express the number of atoms bound together, rather than a count of molecules. Thus, common chemical conventions apply to the definition of the constituent entities of a substance, in other cases exact definitions may be specified. The mass of a substance is equal to its relative atomic (or molecular) mass multiplied by the molar mass constant, which is almost exactly 1 g/mol.
History
The history of the mole is intertwined with that of units of molecular mass, and the Avogadro constant.
The first table of
Charles Frédéric Gerhardt (1816–56), Henri Victor Regnault (1810–78) and Stanislao Cannizzaro (1826–1910) expanded on Berzelius' works, resolving many of the problems of unknown stoichiometry of compounds, and the use of atomic masses attracted a large consensus by the time of the Karlsruhe Congress (1860). The convention had reverted to defining the atomic mass of hydrogen as 1, although at the level of precision of measurements at that time – relative uncertainties of around 1% – this was numerically equivalent to the later standard of oxygen = 16. However the chemical convenience of having oxygen as the primary atomic mass standard became ever more evident with advances in analytical chemistry and the need for ever more accurate atomic mass determinations.
The name mole is an 1897 translation of the German unit Mol, coined by the
Standardization
Developments in mass spectrometry led to the adoption of oxygen-16 as the standard substance, in lieu of natural oxygen.[10]
The oxygen-16 definition was replaced with one based on carbon-12 during the 1960s. The mole was defined by International Bureau of Weights and Measures as "the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12." Thus, by that definition, one mole of pure 12C had a mass of exactly 12 g.[11][5] The four different definitions were equivalent to within 1%.
Scale basis | Scale basis relative to 12C = 12 |
Relative deviation from the 12C = 12 scale |
---|---|---|
Atomic mass of hydrogen = 1 | 1.00794(7) | −0.788% |
Atomic mass of oxygen = 16 | 15.9994(3) | +0.00375% |
Relative atomic mass of 16O = 16 | 15.9949146221(15) | +0.0318% |
Because a dalton, a unit commonly used to measure atomic mass, is exactly 1/12 of the mass of a carbon-12 atom, this definition of the mole entailed that the mass of one mole of a compound or element in grams was numerically equal to the average mass of one molecule or atom of the substance in daltons, and that the number of daltons in a gram was equal to the number of elementary entities in a mole. Because the mass of a nucleon (i.e. a proton or neutron) is approximately 1 dalton and the nucleons in an atom's nucleus make up the overwhelming majority of its mass, this definition also entailed that the mass of one mole of a substance was roughly equivalent to the number of nucleons in one atom or molecule of that substance.
Since the definition of the gram was not mathematically tied to that of the dalton, the number of molecules per mole NA (the Avogadro constant) had to be determined experimentally. The experimental value adopted by
The mole was made the seventh SI base unit in 1971 by the 14th CGPM.[14]
2019 redefinition of SI base units
Before the 2019 redefinition of the SI base units, the mole was defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 12 grams of carbon-12 (the most common isotope of carbon).[15] The term gram-molecule was formerly used to mean one mole of molecules, and gram-atom for one mole of atoms.
In 2011, the 24th meeting of the General Conference on Weights and Measures (CGPM) agreed to a plan for a possible revision of the SI base unit definitions at an undetermined date.
On 16 November 2018, after a meeting of scientists from more than 60 countries at the CGPM in Versailles, France, all SI base units were defined in terms of physical constants. This meant that each SI unit, including the mole, would not be defined in terms of any physical objects but rather they would be defined by physical constants that are, in their nature, exact.[3]
Such changes officially came into effect on 20 May 2019. Following such changes, "one mole" of a substance was redefined as containing "exactly 6.02214076×1023 elementary entities" of that substance.[18][19]
Criticism
Since its adoption into the International System of Units in 1971, numerous criticisms of the concept of the mole as a unit like the metre or the second have arisen:
- the number of molecules, etc. in a given amount of material is a fixed dimensionless quantity that can be expressed simply as a number, not requiring a distinct base unit;[5][20]
- The SI thermodynamic mole is irrelevant to analytical chemistry and could cause avoidable costs to advanced economies[21]
- The mole is not a true metric (i.e. measuring) unit, rather it is a parametric unit, and amount of substance is a parametric base quantity[22]
- the SI defines numbers of entities as quantities of dimension one, and thus ignores the ontological distinction between entities and units of continuous quantities[23]
In chemistry, it has been known since
Similar units
Like chemists, chemical engineers use the unit mole extensively, but different unit multiples may be more suitable for industrial use. For example, the SI unit for volume is the cubic metre, a much larger unit than the commonly used litre in the chemical laboratory. When amount of substance is also expressed in kmol (1000 mol) in industrial-scaled processes, the numerical value of molarity remains the same, as . Chemical engineers once used the kilogram-mole (notation kg-mol), which is defined as the number of entities in 12 kg of 12C, and often referred to the mole as the gram-mole (notation g-mol), then defined as the number of entities in 12 g of 12C, when dealing with laboratory data.[25]
Late 20th-century chemical engineering practice came to use the kilomole (kmol), which was numerically identical to the kilogram-mole (until the 2019 redefinition of SI units, which redefined the mole by fixing the value of the Avogadro constant, making it very nearly equivalent to but no longer exactly equal to the gram-mole), but whose name and symbol adopt the SI convention for standard multiples of metric units – thus, kmol means 1000 mol. This is equivalent to the use of kg instead of g. The use of kmol is not only for "magnitude convenience" but also makes the equations used for modelling chemical engineering systems coherent. For example, the conversion of a flowrate of kg/s to kmol/s only requires dividing by the molar mass in g/mol (as ) without multiplying by 1000 unless the basic SI unit of mol/s were to be used, which would otherwise require the molar mass to be converted to kg/mol.
For convenience in avoiding conversions in the
Greenhouse and growth chamber lighting for plants is sometimes expressed in micromoles per square metre per second, where 1 mol photons ≈ 6.02×1023 photons.[26] The obsolete unit einstein is variously defined as the energy in one mole of photons and also as simply one mole of photons.
Derived units and SI multiples
The only
Submultiples | Multiples | ||||
---|---|---|---|---|---|
Value | SI symbol | Name | Value | SI symbol | Name |
10−1 mol | dmol | decimole | 101 mol | damol | decamole |
10−2 mol | cmol | centimole | 102 mol | hmol | hectomole |
10−3 mol | mmol | millimole | 103 mol | kmol | kilomole |
10−6 mol | μmol | micromole | 106 mol | Mmol | megamole |
10−9 mol | nmol | nanomole | 109 mol | Gmol | gigamole |
10−12 mol | pmol | picomole | 1012 mol | Tmol | teramole |
10−15 mol | fmol | femtomole | 1015 mol | Pmol | petamole |
10−18 mol | amol | attomole | 1018 mol | Emol | examole |
10−21 mol | zmol | zeptomole | 1021 mol | Zmol | zettamole |
10−24 mol | ymol | yoctomole | 1024 mol | Ymol | yottamole |
10−27 mol | rmol | rontomole | 1027 mol | Rmol | ronnamole |
10−30 mol | qmol | quectomole | 1030 mol | Qmol | quettamole |
One femtomole is exactly 602,214,076 molecules; attomole and smaller quantities cannot be exactly realized. The yoctomole, equal to around 0.6 of an individual molecule, did make appearances in scientific journals in the year the yocto- prefix was officially implemented.[27]
Mole Day
October 23, denoted 10/23 in the US, is recognized by some as Mole Day.[28] It is an informal holiday in honor of the unit among chemists. The date is derived from the Avogadro number, which is approximately 6.022×1023. It starts at 6:02 a.m. and ends at 6:02 p.m. Alternatively, some chemists celebrate June 2 (06/02), June 22 (6/22), or 6 February (06.02), a reference to the 6.02 or 6.022 part of the constant.[29][30][31]
See also
- Element-reactant-product table
- Faraday constant – Physical constant: Electric charge of one mole of electrons
- Mole fraction – Proportion of a constituent in a mixture
- Dalton (unit) – Standard unit of mass for atomic-scale chemical species
- Molecular mass – Mass of a given molecule in daltons
- Molar mass – Mass per amount of substance
References
- ^ ISBN 978-92-822-2272-0
- ^ S2CID 241546445.
- ^ a b "On the revision of the International System of Units – International Union of Pure and Applied Chemistry". International Union of Pure and Applied Chemistry. 16 November 2018. Retrieved 1 March 2021.
- ^ BIPM (20 May 2019). "Mise en pratique for the definition of the mole in the SI". BIPM.org. Retrieved 18 February 2022.
- ^ .
- ^ Helm, Georg (1897). The Principles of Mathematical Chemistry: The Energetics of Chemical Phenomena. transl. by Livingston, J.; Morgan, R. New York: Wiley. p. 6.
- molecular weight).
- ^ Ostwald, Wilhelm (1893). Hand- und Hilfsbuch zur Ausführung Physiko-Chemischer Messungen [Handbook and Auxiliary Book for Conducting Physico-Chemical Measurements]. Leipzig, Germany: Wilhelm Engelmann. p. 119. From p. 119: "Nennen wir allgemein das Gewicht in Grammen, welches dem Molekulargewicht eines gegebenen Stoffes numerisch gleich ist, ein Mol, so ... " (If we call in general the weight in grams, which is numerically equal to the molecular weight of a given substance, a "mol", then ... )
- ^ mole, n.8, Oxford English Dictionary, Draft Revision Dec. 2008
- ^ Busch, Kenneth (May 2, 2003). "Units in Mass Spectrometry" (PDF). Current Trends in Mass Spectrometry. 18 (5S): S32-S34 [S33]. Retrieved 29 April 2023.
- ^ ISBN 92-822-2213-6, archived(PDF) from the original on 2021-06-04, retrieved 2021-12-16
- ^ physics.nist.gov/ Archived 2015-06-29 at the Wayback Machine Fundamental Physical Constants: Avogadro Constant
- S2CID 18291648.
- ^ "BIPM – Resolution 3 of the 14th CGPM". www.bipm.org. Archived from the original on 9 October 2017. Retrieved 1 May 2018.
- ISBN 92-822-2213-6, archived(PDF) from the original on 2021-06-04, retrieved 2021-12-16
- ^
Wang, Yuxing; Bouquet, Frédéric; Sheikin, Ilya; Toulemonde, Pierre; Revaz, Bernard; Eisterer, Michael; Weber, Harald W.; Hinderer, Joerg; Junod, Alain; et al. (2003). "Specific heat of MgB2 after irradiation". Journal of Physics: Condensed Matter. 15 (6): 883–893. S2CID 16981008.
- S2CID 38571250.
- ^ CIPM Report of 106th Meeting Archived 2018-01-27 at the Wayback Machine Retrieved 7 April 2018
- ^ "Redefining the Mole". NIST. 2018-10-23. Retrieved 24 October 2018.
- .
- S2CID 95388009.
- S2CID 122242959.
- S2CID 46532636.
- OL 7204743M.
- ^ ISBN 978-0-13-305798-0.
- ^ "Lighting Radiation Conversion". Archived from the original on March 11, 2016. Retrieved March 10, 2016.
- PMID 1761625.
- ^ History of National Mole Day Foundation, Inc. Archived 2010-10-23 at the Wayback Machine.
- ^ Happy Mole Day! Archived 2014-07-29 at the Wayback Machine, Mary Bigelow. SciLinks blog, National Science Teachers Association. October 17, 2013.
- ^ What Is Mole Day? – Date and How to Celebrate. Archived 2014-07-30 at Wikiwix, Anne Marie Helmenstine. About.com.
- ^ The Perse School (Feb 7, 2013), The Perse School celebrates moles of the chemical variety, Cambridge Network, archived from the original on 2015-02-11, retrieved Feb 11, 2015,
As 6.02 corresponds to 6th February, the School has adopted the date as their 'Mole Day'.
External links
- ChemTeam: The Origin of the Word 'Mole' at the Wayback Machine (archived December 22, 2007)