Question 12
What is the chemical formula of ammonium phosphate?
The right answer is the one marked above.
(NH4)3PO4. The ammonium ion NH4+ has a valency of 1 and the phosphate ion PO43− a valency of 3, so crossing over needs three ammonium ions to one phosphate.
Both parts here are polyatomic ions, and only the one that repeats takes brackets — three ammonium groups, one phosphate. NH4(PO4)3 has them the wrong way round.