# Redefinition of mole 2019

The mole was redefined in 2019 to $$6.02214076\times 10^{23}$$ elementary entities. I've learnt that this means that the mass of one mole of Carbon 12 is now no longer exactly 12g. However isn't Avagadros number or $$6.02214076\times 10^{23}$$ a measure of the number of atoms in 12g of Carbon 12? It seem this is no longer the case, so how exactly is Avagadros number now calculated?

• The Avogadro constant $$N_A$$ was the approximate number (as far as measurement allowed to know) of carbon-12 atoms in $$\pu{12 g}$$ of pure carbon-12 isotope.
• Now $$N_A$$ is $$\pu{6.02214076×10^23 mol-1}$$ by definition, i.e. it is not calculated.
Therefore, $$\pu{1 mol} = \pu{6.02214076×10^23}$$ atoms of carbon-12 does not have exactly the mass $$\pu{12 g}$$.
It relates the molar mass constant $$M_\mathrm{u}$$ and the atomic mass constant $$m_\mathrm{u}$$ currently $$\pu{1.66053906660(50)E−27 kg}$$:
$$M_\mathrm{u} = m_\mathrm{u} N_\mathrm{A} = \pu{0.99999999965(30)E−3 kg⋅mol−1}$$