extent of reaction, $$\xi$$

https://doi.org/10.1351/goldbook.E02283
extensive quantity describing the progress of a chemical reaction equal to the number of chemical transformations, as indicated by the reaction equation on a molecular scale, divided by the Avogadro constant (it is essentially the amount of chemical transformations). The change in the extent of reaction is given by dξ = dnB/νB, where νB is the stoichiometric number of any reaction entity B (reactant or product) and dnB is the corresponding amount.
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