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The mole (symbol mol) is the unit of amount of substance in the International System of Units (SI), used to denote the quantity amount of substance is a measure of how many elementary entities of a given substance are in an object or sample.
The mole is defined as containing exactly 6.02214076×1023 elementary entities. The number of elementary entities in one mole is known as the Avogadro number.
Depending on what the substance is, an elementary entity may be an atom, a molecule, an ion, an ion pair or even a subatomic particle such as an electron. For example, 10 moles of carbon (a chemical element) and 10 moles of water (a chemical compound), contain equal amounts of substance and the carbon contains exactly one atom for each molecule of the water, despite the two having different volumes and different masses.
The mole is widely used in chemistry as a simple way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2 H2 + O2 → 2 H2O can be interpreted to mean that for each 2 mol dihydrogen (H2) and 1 mol dioxygen (O2) that react, 2 mol of water (H2O) form. The concentration of a solution is commonly expressed by its molar concentration, defined as the amount of dissolved substance per unit volume of solution, for which the unit typically used is moles per litre (mol/L).