Balance the following equation by oxidation number method:
$straight P space plus space HNO subscript 3 space space rightwards arrow space space space HPO subscript 3 space plus space NO space plus space straight H subscript 2 straight O$

Solution

(i) The skeleton equation along with oxidation number of each atom is,
0 + 1 +5 -2 +1 +5 -2 +2 -2 +1 -2
P +H N O₃ $rightwards arrow$ H P O₃ N O +H₂ O
(ii) Noting the change in O.N. of above atoms,
Increase P = 0 to 5 i.e. 5
Decrease N = +5 to -2 i.e. 3
(iii) Equalise the increase/decrease in O.N. by multiplying P by 3 and HNO₃ by 5.
3P + 5HNO₃ $rightwards arrow$ HPO₃ + NO + H₂O
(iv) On counting and balancing the atoms on both sides, we get the balanced equation.
3P + 5HNO₃ $rightwards arrow$ 3HPO₃ + 5NO + H₂O