Aluminium sulfate is Al2(SO4)3. Using Al = 26.98, S = 32.06 and O = 16.00 amu, what is its formula mass?

1. Rewrite Al2(SO4)3 in the simpler form Al2S3O12 so that every atom can be counted directly. 2. The subscript 3 outside the bracket multiplies both the sulfur and the four oxygen atoms inside it. 3. Aluminium contributes 2 x 26.98 amu = 53.96 amu. 4. Sulfur contributes 3 x 32.06 amu = 96.18 amu. 5. Oxygen contributes 12 x 16.00 amu = 192.00 amu. 6. The total is 53.96 + 96.18 + 192.00 = 342.14 amu. 7. The value 150.14 amu omits oxygen entirely. 8. The value 214.14 amu counts only 4 oxygen atoms, forgetting to multiply through by the bracket subscript. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.1 Formula Mass and the Mole Concept_