Question

The oxidation number of $P$ in $H_2{P}_2{O}_7$ ion is______.

Answer 1

Answer: 5

In a neutral molecule, the sum of the oxidation numbers of all atoms is zero. Hydrogen has $+1$ oxidation state and oxygen has $-2$ oxidation state. The given compound is pyrophosphoric acid.
$\left[\begin{array}{ccc}+1& x& -2\\ H_2& P_2& O_7\end{array}\right]$
$\therefore \left(+1\times 2\right)+\left(2x\right)+\left(-2\times 7\right)=0$
$x=+5$