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{{#invoke:Hatnote|hatnote}} {{#invoke:redirect hatnote|redirect}} {{#invoke:Infobox|infobox}} The mole is a unit of measurement for amount of substance. It is defined as the amount of any chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with relative atomic mass 12 by definition. This number is expressed by the Avogadro constant, which has a value of {{safesubst:#invoke:val|main}}. In other words, the mole is the name given to an amount of a substance equal in mass (in grams) to the combined mass (in amu) of the atoms of the constituent molecules of the substance multiplied by Avogadro's number. It is one of the base units in the International System of Units; it has 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 2 H2 + O2 → 2 H2O implies that 2 mol of dihydrogen (H2) and 1 mol of dioxygen (O2) react to form 2 mol of water (H2O). The mole may also be used to express the number of atoms, ions, or other elementary entities in a given sample of any substance. The concentration of a solution is commonly expressed by its molarity, defined as the number of moles of the dissolved substance per litre of solution.

The number of molecules in a mole (known as Avogadro's constant) is defined such that the mass of one mole of a substance, expressed in grams, is equal to the mean molecular mass of the substance. For example, the mean molecular mass of natural water is about 18.015, therefore, one mole of water has a mass of about 18.015 grams.

The term gram-molecule was formerly used for essentially the same concept.<ref name="SI114-15">{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref> The term gram-atom has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number of atoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgB2 is 1 gram-molecule of MgB2 but 3 gram-atoms of MgB2.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

In honour of the unit, some chemists celebrate October 23 (a reference to the 1023 part of the Avogadro constant) as "Mole Day". Some also do the same for February 6 and June 2.


Mole (unit) sections
Intro   Definition and related concepts   History  The mole as a unit  Other units called \"mole\"   Proposed future definition   Related units  The unit's holiday   See also    Notes and references    External links   

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