Molar mass constant

The molar mass constant, usually denoted by Mu, is a physical constant defined as the ratio of the molar mass of an element(or a compound) and its relative mass. The molar mass constant, usually denoted by Mu, is a physical constant defined as the ratio of the molar mass of an element(or a compound) and its relative mass. 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 SI unit of atomic mass (dalton). Thus, for example the average atomic mass of chlorine is approximately 35.446 daltons, and the mass of one mole of chlorine atoms was approximately 35.446 grams. On May 20, 2019, the SI definition of mole changed in such a way that the molar mass constant is not exactly 1 g/mol anymore. 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. The molar mass constant is important in writing dimensionally correct equations. While one may informally say 'the molar mass of an element M is the same as it 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. The molar mass constant was unusual (but not unique) among physical constants by having an exactly defined value rather than being measured experimentally. From the old definition of the mole, the molar mass of carbon 12 was exactly 12 g/mol. From the definition of relative atomic mass, the relative atomic mass of carbon 12, that is the atomic weight of a sample of pure carbon 12, is exactly 12. The molar mass constant was thus given by M u = M o l a r   m a s s   [ M ( 12 C ) ] R e l a t i v e   a t o m i c   w e i g h t   [ A r ( 12 C ) ] = 12   g / m o l 12 = 1   g / m o l {displaystyle { m {M_{ m {u}}={{ m {Molar mass }} over { m {Relative atomic weight }}}={{12 { m {g/mol}}} over 12}=1 { m {g/mol}}}}} The speed of light is another example of a physical constant whose value is fixed by the definitions of the International System of Units (SI).