The IUPAC Gold Book defines

Amount concentration

Amount of a constituent divided by the volume of the mixture. Also called amount-of-substance concentration, substance concentration (in clinical chemistry) and in older literature molarity. For entities B it is often denoted by B. The common unit is mole per cubic decimetre or mole per litre sometimes denoted by M.


Number concentration

Number of entities of a constituent in a mixture divided by the volume of the mixture.

I can't see a distinction between the two terms since it seems to me that "Amount of a constituent" is the same as "Number of entities of a constituent".

Could you please explain the difference between them? Is it a conceptual difference or just a difference in the context in which they are most commonly used?


1 Answer 1


The "amount" of a substance is the proper term for the "number of moles" (Wikipedia; Gold Book). So, the two quantities are related by a factor of the Avogadro constant, $\pu{6.022\times 10^23 mol-1}$.

If you have $\pu{1 mol}$ of helium in a $\pu{1 m3}$ box, the amount concentration is $\pu{1 mol m-3}$, or $\pu{0.001 mol dm-3}$ if you prefer those units.

The corresponding number concentration is $\pu{6.022\times 10^23 m-3}$.

  • $\begingroup$ Isn't mol just a specific unit for amount? According to the International System of Units, amount of substance is a quantity, while mol is a unit. I think one can use other units for amount if one wishes: tens, thousands, pi, dozens, etc. $\endgroup$
    – toliveira
    Jul 26, 2018 at 11:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.