What Is a Mole in Chemistry?

A mole is the SI unit for the amount of substance. One mole contains exactly $6.02214076 \times 10^{23}$ specified elementary entities, such as atoms, molecules, ions or formula units.

Why Chemists Use the Mole

Atoms and molecules are too small to count individually in laboratory samples. The mole connects the microscopic number of particles to measurable macroscopic quantities such as mass and solution volume.

The number of particles in one mole is the Avogadro constant:

$$N_{\text{A}} = 6.02214076 \times 10^{23}\ \text{mol}^{-1}$$

The essential relationships in Structure 1.4 are:

Here, $n$ is amount in moles, $N$ is the number of entities, $m$ is mass, $M$ is molar mass, and $c$ is molar concentration.

These equations form a conversion chain: particles connect to moles through the Avogadro constant, and mass connects through molar mass. Moles connect to reacting quantities through balanced-equation coefficients.

For a solution example, 25.0 cubic centimetres of 0.200 molar sodium chloride first converts to 0.0250 cubic decimetres. Therefore, $n=cV=0.200\times0.0250=5.00\times10^{-3}\ \text{mol}$. This demonstrates the unit rule: when concentration is measured in moles per cubic decimetre, volume must be expressed in cubic decimetres. Multiplying the amount by the Avogadro constant then gives the number of sodium chloride formula units.

For example, water has a molar mass of approximately $18.0\ \text{g mol}^{-1}$. Therefore:

$$n = \frac{18.0\ \text{g}}{18.0\ \text{g mol}^{-1}} = 1.00\ \text{mol}$$

This sample contains $6.022 \times 10^{23}$ water molecules. Because each water molecule contains three atoms, it contains three times as many atoms overall.

A common misconception is that a mole is a mass. It is not: a mole measures an amount of entities. One mole of different substances has the same number of entities but usually a different mass.

IB Exam Technique

This content is assessed at both SL and HL. Always identify the specified entity, write the correct relationship, include units, and distinguish atoms, molecules, ions and formula units when giving a final answer.