mole (unit)

mole (unit)
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Name	mole (unit)
Quantity	amount of substance
SI status	base unit
Defined	2019 redefinition of SI
Defining constant	Avogadro constant
Value	6.02214076×10^23 mol^−1

mole (unit) The mole is the SI base unit for amount of substance, defined by a fixed numerical value of the Avogadro constant. It provides the link between the microscopic scale of atoms and molecules and macroscopic measurements used in laboratories and industry. The unit underpins quantitative work in analytical chemistry, thermodynamics, and metrology.

Definition and SI status

Since the 2019 redefinition of the SI, the mole is defined by fixing the numerical value of the Avogadro constant to 6.02214076×10^23 when expressed in the unit mol^−1. This definition aligns the mole with the International System of Units reform spearheaded by bodies such as the General Conference on Weights and Measures, the International Committee for Weights and Measures, and the Bureau International des Poids et Mesures. The mole is listed among the seven SI base units alongside the metre, kilogram, second, ampere, kelvin, and candela, and its formal status is maintained in SI Brochures and international standards developed by organizations like the International Organization for Standardization.

History and development

The concept of a counting unit for entities in chemistry evolved through the 19th and 20th centuries with contributions from figures and institutions such as Amedeo Avogadro, the Royal Society of London, the German Chemical Society, and researchers associated with the Royal Society of Chemistry. Early formulations of atomic and molecular hypothesis—debated in venues including the Congress of Italian Scientists and influenced by chemists like John Dalton and Jean-Baptiste Dumas—led to proposals for a chemical equivalent of the dozen. The term "mole" entered modern usage via 19th‑century German chemistry and was codified in metrological practice through committees of the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics, culminating in the 1971 adoption of the mole as an SI base unit and its 2018–2019 redefinition driven by advances in quantum metrology at labs such as the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt.

Avogadro constant and numerical value

The Avogadro constant expresses the number of specified elementary entities per mole; its historical origins trace to Avogadro's hypothesis and experimental determinations influenced by methods developed by scientists associated with the Royal Institution, the Cavendish Laboratory, and the Institut Laue–Langevin. The fixed numerical value 6.02214076×10^23 mol^−1 was chosen based on high-precision measurements from determinations using the X‑ray crystal density method applied to silicon spheres produced by collaborations including the International Avogadro Coordination and national metrology institutes such as the National Research Council (Canada), Laboratoire national de métrologie et d'essais, and NMI Australia. These values tie into constants such as the Boltzmann constant and the Planck constant within the revised SI framework promoted by the CIPM.

Practical realization and measurement

Realizing the mole in practice involves primary methods that count or relate to a known number of entities. Techniques include X‑ray crystal density measurements of nearly perfect silicon spheres produced by industrial and research partners like Crystec and characterized at facilities such as the European Synchrotron Radiation Facility; atom‑counting experiments by teams at the National Institute of Standards and Technology; and coulometric and spectroscopic approaches developed at institutes including the National Physical Laboratory (UK) and the Physikalisch-Technische Bundesanstalt. Traceability chains reference the SI through intercomparisons coordinated by the BIPM and regional metrology organizations such as the EURAMET and APMP.

Applications in chemistry and related fields

The mole is central to stoichiometry taught in curricula at institutions like the Massachusetts Institute of Technology, University of Cambridge, and University of Tokyo and applied in laboratory practice from academic research at the Max Planck Society to industrial processes at firms such as BASF and DuPont. It enables conversion between mass and particle count for compounds in pharmaceutical development at organizations like Pfizer and Roche, petrochemical engineering at companies such as ExxonMobil, and materials science research at centers including the Lawrence Berkeley National Laboratory. In physical chemistry and thermodynamics, the mole appears in the ideal gas law used in work at the Nobel Prize‑winning laboratories and in statistical mechanics frameworks developed by researchers affiliated with institutions like the Institute for Advanced Study.

Units, symbols, and common misconceptions

The internationally recognized symbol for the unit is "mol", recommended by the BIPM and incorporated in standards from the ISO. Common misconceptions include treating the mole as a mass unit (confused with the kilogram) or as an abstract concentration unit; the mole specifically counts entities, similar in concept to the dozen but on a vastly larger scale. Educational reforms and textbooks from publishers such as Oxford University Press and McGraw‑Hill address these misconceptions, while professional guidelines from the IUPAC clarify notation, the use of molarity, and distinctions between amount of substance and related quantities used in laboratory reports at institutions like the American Chemical Society.

Category:SI units