Amount concentration versus number concentration

Question

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.

and

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?

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 $\pu{6.022\times 10^23 m-3}$.