top of page

 G8.  THE MOLE IS NOT A COUNTING UNIT 

As a consequence of the adoption in S.I. of the MOLE as the unit for the physical quantity ‘AMOUNT OF SUBSTANCE’, the ‘mole’ cannot correctly be used in the same sense as
a counting unit like ‘dozen'.

The ‘mole’ is NOT a counting unit:

it is the S.I. unit for the physical quantity known as ‘amount of substance’.

As Peter G. Nelson (Hull) has emphasised so correctly on several occasions:

it is legitimate to write

but incorrect to assert

One can write either

or 

number of eggs = 3 doz = 36

number of molecules = 18.0

x

10

23

= 1.80

x

10

24

number of molecules = 3 mol = 18.0

x

10

23

amount of substance = 3 mol

Using the commonly encountered SI quantity symbols makes the situation clearer still:

N = 18.0

x

10

23

= 1.80

x

10

24

while

n = 3 mol

And, of course, N and n are connected by a proportionality constant L or 

N

A

known as the Avogadro constant.

Sometimes it is necessary to calculate the number of chemical entities in a given amount of substance, particularly as this is a popular style of examination question.

'Chemical entity' may also be seen referred to as just entity, elementary entity, or particle, or elementary particle.  But if not implied by context, an unambiguous chemical formula or sub-atomic particle must be referred to. For the most part we are talking about molecules, ions, atoms, formula unit of a specified chemical entity, or electrons .

bottom of page