Question

Oxidation number of $P$ in $P_2{O}_7^{4-}$ is :

Answer 1

Let the oxidation number of $P$ in $P_2{O}_7^{4-}$ be $x$.
Since the overall charge on the complex is $-4$, the sum of oxidation states of all elements in it should be equal to $-4$.
Therefore, $2x+7(-2)=-2$
or, $x=+5$