The Mole Concept

A balance in a school laboratory can tell you that a strip of magnesium weighs 0.1 g. What it cannot tell you, and what a chemist actually wants to know, is how much magnesium that is in the sense a chemical equation cares about. Equations are recipes written in amounts: two of these react with one of those. Balances are built to read grams. Somewhere between those two facts there has to be a translator, and the mole is it.

This lesson introduces the one unit that lets a laboratory balance speak the language of a chemical equation. You will learn the single sentence the specification uses to define a mole, see why the relative formula mass on a periodic table turns into a weighable quantity the moment you write g after it, and practise moving in both directions between a mass in grams and an amount in moles. By the end you will be able to look at 9.0 g of water and 22 g of carbon dioxide and say, without hesitation, that the two samples hold exactly the same amount of substance.