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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 . 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 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. 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. 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. == Definition and related concepts == , the mole is defined by BIPM to be the amount of substance of a system which contains the same number of elementary entities (e.g. atoms, molecules, ions, electrons) as atoms in 0.012 kilograms of carbon-12 (12C), the isotope of carbon with relative atomic mass 12.〔 Thus, by definition, one mole of pure 12C has a mass of ''exactly'' 12 g. It also follows from the definition that ''X'' moles of any substance will contain the same number of molecules as ''X'' moles of any other substance. The mass per mole of a substance is called its molar mass. Since the standard unit for expressing the mass of molecules or atoms (atomic mass unit or the dalton) is defined as 1/12 of the mass of a 12C atom, it follows that the molar mass of a substance, measured in grams per mole, is exactly equal to its mean molecular or atomic mass, measured in unified atomic mass units or daltons; which is to say, to the substance's mean molecular or relative atomic mass. The number of elementary entities in a sample of a substance is technically called its (chemical) amount. Therefore, the mole is a convenient unit for that physical quantity. One can determine the chemical amount of a known substance, in moles, by dividing the sample's mass by the substance's molar mass.〔International Bureau of Weights and Measures. "(Realising the mole )." Retrieved 25 September 2008.〕 Other methods include the use of the molar volume or the measurement of electric charge.〔 The mass of one mole of a substance depends not only on its molecular formula, but also on the proportion of the isotopes of each element present in it. For example, one mole of calcium-40 is ± grams, whereas one mole of calcium-42 is ± grams, and one mole of calcium with the normal isotopic mix is 40.078 ± 0.004 grams. Since the definition of the gram is not () mathematically tied to that of the atomic mass unit, the number ''N''''A'' of molecules in a mole (Avogadro's number) must be determined experimentally. The value adopted by CODATA in 2010 is ''N''''A'' = ± .〔()〕 In 2011 the measurement was refined to ± . The number of moles in a sample is simply the sample mass divided by the molar mass of the material. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mole (unit)」の詳細全文を読む スポンサード リンク
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