3.2 Determination of formulae

The number of atoms of each element in a molecule Eg. H2, N2, O2, S8

The simplest whole-number ratio of atoms of each element in a compound Ratio is always the same Important for substances that do not exist as molecules, eg. carbon, silicon, sodium chloride, which form giant crystalline structures of atoms or ions

Giant crystalline structures of atoms or ions

Mr, compares the mass of a molecule with the mass of an atom of carbon-12

Add together the relative atomic masses of the elements which make up a molecule

Compares the mass of a formula unit with the mass of an atom of carbon-12 Calculated by adding the atomic masses of the elements in the empirical formula

analysis

If you calculated a ratio as 1:1.67 don’t be tempted to round to 1:2 See if you can convert it into a whole number ratio by multiplying both by the same factor In this case, by 3, so you get a ratio of 3:5

Water molecules that are a part of the crystalline structure of hydrated salts

Lacking water

• Weigh an empty crucible • Add the hydrated salt into the weighed crucible • Weigh the crucible and the hydrated salt • Using a pipe-clay triangle, support the crucible on a tripod, heat it and its contents gently for one minute and then strongly for another three. • Leave it to cool, then weigh

Some assumptions are made, all the water has been lost, and no further decomposition