Phosphate, PO43−, is a polyatomic ion that can combine with any cation to form an ionic compound. Identify all the formulas below that correctly represent phosphate compounds.
Select one or more:
Combined with lithium:
Li3PO4
Combined with iron(III):
Fe3(PO4)3
Combined with iron(III):
FePO4
Combined with copper(I):
Cu3PO4
Combined with copper(I):
(Cu)3PO4
Combined with magnesium:
Mg3(PO4)2
Combined with magnesium:
Mg2(PO4)3
Combined with lithium:
Li2PO4

Li3PO4 - correct

FePO4 - correct

Cu3PO4 - correct

Mg3(PO4)2 - correct

Explanation:

In order to answer this question we must be conversant with the idea of valency as well as the idea of writing formula of ionic substances.

Lithium has a valency of 1 while PO4^3- has a valency of 3 so the correct formula of lithium phosphate must be Li3PO4 after the exchange of valencies.

Iron III has a valency of 3 while the phosphate ion also has a valency of 3 so the subscripts cancel out and iron III phosphate is FePO4.

Copper I ion has a valency of 1 and the phosphate ion has a valency of 3. When they exchange valencies, the formula of the compound is Cu3PO4.

Magnesium has a valency of 2 while the phosphate ion has a valency of 3. The exchange of valency gives us the formula Mg3(PO4)2.