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The mole is a handy way of measuring an amount of a substance in chemistry. It can, however, be confusing because we’re used to measuring amounts of substances using units like mass or length that are independent of the nature of the substance being measured. The mole is not like that.

To understand moles, you need to understand the chemistry of the substance you’re dealing with; whether it is made of atoms or molecules and what the nature of those atoms or molecules is. You might have a lump of pure carbon, say, or a vial of methane. The mole is simply a way of saying how many of those atoms of carbon or molecules of methane you have. One mole is defined as a collection of 6.02 x 10²³ of those atoms or molecules.

This number is known as Avogadro’s constant and it is not as random as it might seem. In fact, it is set so that a mole of any pure substance will be equal in weight to its relative mass. Take carbon, with a relative atomic mass of 12: a mole of it weights 12 grams. A mole of paracetamol, with a relative molecular mass of 151 weighs, you guessed it, 151 grams.

There’s good reason to bother with moles when it comes to stitching together atoms to make new molecules, including drugs, and that’s efficiency. Let’s say a chemist wants to take a chain of carbon atoms and add another carbon to the end. It’s no good simply combining equal masses of the two reactants – that would waste lots of material – what you need to do is combine equal amounts of two components in moles.