What is a mole?

Rather than the furry creature, in chemistry a mole is a unit used to measure substances. Its equivalent to 6x10²³ atoms, the number of carbon atoms in 12g of carbon.

To find a mole of a substance we use the equation Mass=Mr x mol (easily remembered as mass=Mr Mole). We can rearrange this to give Mass/Mr= Mol.

Therefore to find the amount of moles of a substance we need the mass and Mr. The Mr (relative formula mass) can be easily calculated using the relative atomic masses(Ar) of each element, as found in the periodic table. For example the Mr of water, H₂O, is 18. This is because the Ar of hydrogen is 1, and we have 2 hydrogen atoms. The Ar of Oxygen is 16. 16+2=18.

If we have 5g of H₂O, and the Mr is 18

Mol= mass/Mr   mol=5/18 =0.278 (rounded to 3dp)