Why use moles?

A huge problem

An atom is incredibly small: there are thousands of billions of billions of molecules in a single drop of water. Counting atoms or molecules one by one, as one might count apples, is completely impossible in practice. Yet chemists need to know ‘how many’ particles are reacting together in order to write and use a reaction equation.

The idea: counting in bundles

The solution is the same as in everyday life: when there are too many objects to count one by one, we group them into bundles. We count eggs by the dozen, and paper by reams of 500 sheets. In chemistry, the chosen group is called a mole: a mole always contains the same number of particles (atoms, molecules, ions, etc.), regardless of the element or substance in question.

The mole, a unit of amount of substance

The mole is the unit of amount of substance, denoted by n, in the International System of Units. Saying ‘I have 2 moles of iron’ makes just as much sense as saying ‘I have 2 dozen eggs’: it indicates a precise number of particles, without needing to count them one by one.

Practical example

One mole of water molecules (H₂O) contains exactly the same number of molecules as one mole of iron atoms contains atoms: this common number is Avogadro’s number, which you’ll learn about in the next lesson.

Common mistake

Do not confuse the amount of substance (the number of ‘bundles’ of particles, expressed in moles) with mass (expressed in g), which also depends on the nature of the particle. One mole of iron and one mole of water contain the same number of particles, but do not have the same mass at all.