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The mole is a unit of measurement for the amount of substance or chemical amount. It is one of the base units in the International System of Units, and has the unit symbol mol.^{[1]}
The name mole is an 1897 translation^{[2]}^{[3]} of the German unit Mol, coined by the chemist Wilhelm Ostwald in 1893,^{[4]} although the related concept of equivalent mass had been in use at least a century earlier. The name is derived^{[5]} from the German word Molekül (molecule).
The mole is defined as the amount of substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 g of the isotope carbon12 (^{12}C).^{[1]} Thus, by definition, one mole of pure ^{12}C has a mass of exactly 12 g. The amount of substance n (in mol) and the number of atoms or molecules contained in it, N, are proportional, and the proportionality constant N/n is known as the Avogadro constant. By definition it has the dimension of the inverse of amount of substance (unit mol^{−1}) and its experimentally determined value is 6.02214179(30)×1023 mol^{−1}.^{[6]}
By this definition, a mole of any pure substance has a mass in grams exactly equal to that substance's molecular or atomic mass; e.g., 1mol of calcium40 is approximately equal to 40g, because the Ca40 isotope has a mass of 39.9625906 amu on the C12 scale. In other words, the numerical value of a substance's molecular or atomic mass in atomic mass units is the same as that of its molar mass—the mass of one mole of that substance—in grams.
The most common method of determining the amount, expressed in moles, of pure substance the value of whose molar mass is known, is to measure its mass in grams and then to divide by its molar mass (expressed in g/mol).^{[7]} Molar masses may be easily calculated from tabulated values of atomic weights and the molar mass constant (which has a convenient defined value of 1 g/mol). Other methods include the use of the molar volume or the measurement of electric charge.^{[7]}
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