As of 2017IUPAC, the mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in mol−1, and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, or any other particle or specified group of particles.
as defined by
The molar mass of a substance is its mass divided by its amount of substance, which is a constant for any given substance. Since the unified atomic mass unit (symbol: u, or Da) is defined as 1/12 of the mass of the 12C atom, it follows that the molar mass of a substance, measured in grams per mole, is numerically equal to its mean atomic or molecular mass measured in Da.
The number of constitutive entities in a sample of a substance is technically called its (chemical) amount. Therefore, the mole is a 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. 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 ±0.00000022 grams, whereas one mole of 39.96259098calcium-42 is ±0.00000027 grams, and one mole of 41.95861801calcium with the normal isotopic mix is ±0.004 grams. 40.078
Since the definition of the gram is not (as of 2011NA (the Avogadro constant) must be determined experimentally. The value adopted by CODATA in 2010 is NA = ±0.00000027)×1023 mol−1. (6.02214129 In 2011 the measurement was refined to ±0.00000018)×1023 mol−1. (6.02214078
) mathematically tied to that of the atomic mass unit, the number of molecules per mole
The number of moles of a sample is the sample mass divided by the molar mass of the material.