The Mole and Avogadro's Constant

This lesson introduces the concept of the mole — the chemist's counting unit — as required by the AQA GCSE Combined Science Trilogy specification (8464). You will learn what a mole is, understand Avogadro's constant, and be able to convert between mass, moles and relative formula mass.

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Why Do Chemists Need the Mole?

Atoms are incredibly small. A single carbon atom has a mass of about $2 \times 10^{-23}$2×10−23 g — far too small to weigh individually. Chemists need a way to count atoms by weighing substances, and the mole provides that bridge.

The mole connects the submicroscopic world (atoms and molecules) to the macroscopic world (grams and kilograms on a balance).

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What Is One Mole?

One mole of any substance contains exactly $6.022 \times 10^{23}$6.022×1023 particles (atoms, molecules, ions or formula units). This number is called Avogadro's constant ($N_A$NA​).

$N_A = 6.022 \times 10^{23} \text{ mol}^{-1}$NA​=6.022×1023 mol−1

Putting This Number in Perspective