Calculate the oxidation number of phosphorous in H_{3}PO_{4}.

let us consider the oxidation number of phosphorous to be x.

also, we know that the oxidation number of hydrogen is +1

the oxidation number of oxygen is -2.

(+1) × 3 + x + (-2 ) × 4 = 0

+3 + x – 8 = 0

x – 5 = 0

x = 0

**Hence the oxidation number of P in H _{3}PO_{4} is + 5.**