What is the oxidation number of aluminium in potash alum?

The oxidation number of aluminium in potash alum is +3. This is because in the compound potassium aluminum sulfate dodecahydrate (KAl(SO4)2·12H2O), aluminum is bonded to two sulfate ions (each with a charge of -2) and one potassium ion (with a charge of +1), resulting in a total oxidation state of +3 for aluminium.