Chemistry - Complete d-block Question with Solution | TestHub

Phosphomolybdate is an anion containing both phosphorus and molybdenum, with the chemical formula ${\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4·12{\mathrm{MoO}}_3$. 

The ammonium ion has a total charge of $+3$, and the phosphate ion has a total charge of $-3$. Therefore, the $12 {\mathrm{MoO}}_3$ units in the compound must have a total charge of $+3 - (-3) = +6$.

Each ${\mathrm{MoO}}_3$ unit contains one $\mathrm{Mo}$ atom and three $\mathrm{O}$ atoms. The total oxidation state of the three $\mathrm{O}$ atoms is $-6$, since each $\mathrm{O}$ atom has an oxidation state of $-2$.

So, 

$$\begin{gathered} \mathrm{Mo} + (-6) = 0 \\ \mathrm{Mo} = +6 \end{gathered}$$