The mole

When dealing with practical chemistry we are always involved with large numbers of particles. Single atoms and molecules are inconveniently small. The mole is the unit of amount of substance that we use. The strict definition is:

The mole is the amount of substance which contains the same number of elementary particles as there are in exactly 12 grams of carbon-12.

This means that the mole is a very large number of particles. The precise number is known as the Avagadro constant and is around 6.02 × 10²³ or 602,000,000,000,000,000,000,000. That is six hundred and two thousand million million million!

A mole of any substance is simply the relative mass in grams. We can get an approximate value for the relative mass of a molecule by adding up the molar masses for all the atoms in the substance. The molar masses given on your periodic table are only approximate, but are sufficiently accurate for the work that we do. The molar mass is the larger of the two numbers found on your periodic table. For example, water has the formula H₂O, with the masses H =1 and O = 16. So the molar mass of water is 2 × 1 + 16 = 18 g mol^-1.

A mole of copper (63.5 g) with a 1p piece for scale

Exercise

Try to work out the mass of one mole of each of the following substances. You can check your answer by clicking in the answer box.

Formula	Molar mass (g mol^-1)	Formula	Molar mass (g mol^-1)
NaCl		H₂SO₄
Ca(OH)₂		(NH₄)₂SO₄
C₁₂H₂₂O₁₁		CuSO₄.5H₂O

We often use fractions of a mole in equations. Don't be fooled by this. ½O₂ does not mean that we've split an O₂ molecule in two. It refers to half a mole of O₂ molecules. Remember that this is still 301,000,000,000,000,000,000,000 O₂ molecules!