How many oxygen atoms does one formula unit of Al2(SO4)3 contain?

1. The sulfate group SO4 carries four oxygen atoms. 2. The subscript 3 outside the bracket applies to the whole group, so there are three sulfate groups. 3. That gives 3 x 4 = 12 oxygen atoms, and the formula can be rewritten as Al2S3O12. 4. Answering four means reading the bracket contents once and ignoring the outer subscript. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_