Back ثابت الكتلة المولية Arabic Molární hmotnostní konstanta Czech Constante de masa molar Spanish Konstanta massa molar ID Costante di massa molare Italian モル質量定数 Japanese

Molar mass constant

The molar mass constant, usually denoted by Mu, is a physical constant defined as one twelfth of the molar mass of carbon-12: Mu = M(12C)/12.[1] The molar mass of any element or compound is its relative atomic mass (atomic weight) multiplied by the molar mass constant.

The mole and the relative atomic mass were originally defined in the International System of Units (SI) in such a way that the constant was exactly 1 g/mol. That is, the numerical value of the molar mass of an element, in grams per mole of atoms, was equal to its atomic mass relative to the atomic mass constant, mu. Thus, for example, the average atomic mass of chlorine is approximately 35.446 daltons, making the mass of one mole of chlorine atoms approximately 35.446 grams.

On 20 May 2019, the SI definition of mole changed in such a way that the molar mass constant remains nearly but no longer exactly 1 g/mol. However, the difference is insignificant for all practical purposes. According to the SI, the value of Mu now depends on the mass of one atom of carbon-12, which must be determined experimentally. As of that date, the 2018 CODATA recommended value of Mu is 0.99999999965(30)×10−3 kg⋅mol−1.[2][3]

The molar mass constant is important in writing dimensionally correct equations.[4] While one may informally say "the molar mass of an element M is the same as its atomic weight A", the atomic weight (relative atomic mass) A is a dimensionless quantity, whereas the molar mass M has the units of mass per mole. Formally, M is A times the molar mass constant Mu.

  1. ^ Barry N Taylor (2009). "Molar mass and related quantities in the New SI". Metrologia. 46.
  2. ^ "2018 CODATA Value: molar mass constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  3. ^ Mohr, Peter J.; Taylor, Barry N. (2005). "CODATA recommended values of the fundamental physical constants: 2002". Rev. Mod. Phys. 77 (1): 1–107. arXiv:1507.07956. Bibcode:2005RvMP...77....1M. doi:10.1103/RevModPhys.77.1.
  4. ^ de Bièvre, Paul; Peiser, H. Steffen (1992). "'Atomic Weight' — The Name, Its History, Definition, and Units" (PDF). Pure and Applied Chemistry. 64 (10): 1535–43. doi:10.1351/pac199264101535.
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