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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?

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  • 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 $\pu{1.66053906660(50)E−27 kg}$:

$M_\mathrm{u} = m_\mathrm{u} N_\mathrm{A} = \pu{0.99999999965(30)E−3 kg⋅mol−1}$

Note that the element molar masses have uncertainty much bigger that the shift due Avogadro number redefinition, because of isotope abundance fluctuation. The exception are elements with a single natural isotope, where it is comparable.

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