**Q. What is the oxidation number of phosphorous in PO _{4} ^{3-}?**

Let us consider the oxidation of phosphorous to be y

Also, The oxidation number of oxygen is -2

here the ion has three negative charges.

y + (-2) × 4 = -3

y – 8 = -3

y=+5

**Hence, the oxidation number of P in PO _{4} ^{3-} is +5.**