Avogadro constant

The Avogadro constant is the number of particles in one mole of any substance, 6.02 x 10²³ mol⁻¹. One mole contains this many atoms, molecules or ions, whatever the substance. Malay: pemalar Avogadro · Chinese: 阿伏伽德罗常数.

This constant is what gives the mole its meaning, because it fixes exactly how many particles “one mole” stands for. It is a very large number precisely because atoms and molecules are so small, so even a tiny mass of a substance contains an enormous count of particles. You do not have to memorise its full value, because it is provided in the exam, but you must know what it represents and how to use it.

Example. To find the number of particles in a sample, multiply the number of moles by the Avogadro constant. Two moles of carbon dioxide therefore contain 2 × 6.02 x 10²³ mol⁻¹ molecules. Working the other way, if you know the number of particles, you divide by the Avogadro constant to find the number of moles.

Confusion to avoid. Do not confuse the Avogadro constant with Avogadro’s law. The constant is a fixed number of particles per mole; Avogadro’s law is a separate statement that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. They share a name but describe different things. Also remember that the constant counts whatever particle the substance is made of, so you must state whether you mean atoms, molecules or ions.

In this chapter the Avogadro constant is the bridge between the amount in moles and the actual number of particles, one of the three quantities the mole links together. Quoting its value and using it correctly in a conversion is a routine source of marks in SPM Chemistry.