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

Weigh you compound, remove one element of the compound through a reaction (break up a metal oxide or salts), then weigh again.

The first weight is AB the second weight is A so doing 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.

e.g

A= 22g with an ar of 11

A= 22/11= 2

B=72g with an ar of 18

B=72/18= 4

A2B4 and to simplify to empirical formula AB2