What is a mole?

In chemistry, quantities of substances are traditionally measured in volume or mass. Thus, the concept of mole (symbol mol) is very important, since it allows stoichiometric calculations in chemical reactions.

In this way, we can indicate what ratio exists between reactants and products.

For example, in the reaction of nitric acid formation, one mole of ammonia reacts with two moles of oxygen to give one mole of nitric acid and one mole of water:

NH3 + 2 O2 → HNO3 + H2O

On the other hand, since the concept of relative mass (atomic weights) was introduced by John Dalton, chemists have been able to express the measurements of their experiments in quantities proportional to the number of elementary entities.

Thus, in the International System of Units (SI), the quantity of a substance is related to the mass of the kilogram.

1971 IUPAC definition The amount of substance, of given chemical formula, containing the same number of formula units (atoms, molecules, ions, quanta, or other entities) as the atoms contained in exactly 12 g of 12C.

Therefore, mole would be the amount of substance of a system containing as many elementary units with atoms as there are in 0.012 kg of carbon-12.

In addition, when we use mol, we must specify what type of elementary units we are referring to.

For example, whether they are atoms, molecules, ions, electrons, other particles or groups of these. That is, if we are referring to a “mole of atoms”, “mole of molecules”, “mole of electrons”, etc.

The 1971 IUPAC definition of mole recommended using 0.012 kg of carbon-12 as a reference. However, nowadays, Avogadro’s number can be determined with great precision. Therefore, this leads us to redefine mol using this constant.

Consequently, recently (2018), the IUPAC has redefined mol, adopting it as the quantity of substance used as the unit of SI. Its definition would remain as:

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 8 × 1023 elementary entities. This number is called the Avogadro number.


Avogadro number elementary entities

This number is fixed to the numerical value of the Avogadro constant, NA, when expressed in mol-1, and is called Avogadro’s number.

Video about Mole Concept