Phosphomolybdate is an anion containing both phosphorus and molybdenum, with the chemical formula (NH4)3PO4⋅12MoO3.
The ammonium ion has a total charge of +3, and the phosphate ion has a total charge of −3. Therefore, the 12MoO3 units in the compound must have a total charge of +3−(−3)=+6.
Each MoO3 unit contains one Mo atom and three O atoms. The total oxidation state of the three O atoms is −6, since each O atom has an oxidation state of −2.
So,
Mo+(−6)=0Mo=+6