Write the formula of ammonium phosphomolybdate

Ammonium phosphomolybdate is commonly represented as: $(NH_4)_3[PMo_{12}O_{40}]$

The oxidation state of Mo is to be found in the anion: $[PMo_{12}O_{40}]^{3-}$

Assign known oxidation states

Let oxidation state of each Mo be $x$.

• Oxidation state of $P = +5$
• Oxidation state of $O = -2$
• There are $12$ Mo atoms and $40$ O atoms.

Set up the oxidation state equation

Total oxidation state of the anion equals its charge $(-3)$: $(+5) + 12x + 40(-2) = -3$

Solve the equation

$5 + 12x - 80 = -3$ $12x - 75 = -3$ $12x = 72$ $x = 6$

Conclusion

Therefore, the oxidation state of Mo is: $+6$