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Approximate definition of a mole based on 12 grams of carbon-12. The Avogadro constant was historically derived from the old definition of the mole as the amount of substance in 12 grams of carbon-12 (12 C); or, equivalently, the number of daltons in a gram, where the dalton is defined as 1 / 12 of the mass of a 12 C atom. [8]
Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons , 6 neutrons , and 6 electrons .
By definition, the atomic mass of carbon-12 is 12 Da, giving a molar mass of 12 g/mol. The number of molecules per mole in a substance is given by the Avogadro constant, exactly 6.022 140 76 × 10 23 mol −1 since the 2019 revision of the SI. Thus, to calculate the stoichiometry by mass, the number of molecules required for each reactant is ...
Alternately, the atomic mass of a carbon-12 atom may be expressed in any other mass units: for example, the atomic mass of a carbon-12 atom is 1.992 646 882 70 (62) × 10 −26 kg. As is the case for the related atomic mass when expressed in daltons , the relative isotopic mass numbers of nuclides other than carbon-12 are not whole numbers, but ...
The molar mass constant, usually denoted by M u, is a physical constant defined as one twelfth of the molar mass of carbon-12: M u = M(12 C)/12. [1] The molar mass of an element or compound is its relative atomic mass (atomic weight) or relative molecular mass (molecular weight or formula weight) multiplied by the molar mass constant.
The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as 1 / 12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. [1] [2] [3] It is a non-SI unit accepted for use with SI.
The International Bureau of Weights and Measures defined the mole as "the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12." Thus, by that definition, one mole of pure 12 C had a mass of exactly 12 g. [15] [5] The four different definitions were equivalent to within 1%.
The molecular mass (abbreviated M r) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of 12 C).