1.23 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation

One way of working out formulae in an experiment is... 

- Weigh your compound
- Remove one element of the compound through a reaction (break up a metal oxide or salts)
- Weigh again

The first weight is AB the second weight is A so do AB-A= B.

Now that you have the weight of A and B you can work out the formulae by doing the weight divided by the Ar.

For example...

A= 22g with an Ar of 11

A= 22/11= 2

B=72g with an Ar of 18

B=72/18= 4

This means the formula is, A2B4 which can simplify to empirical formula AB2