Consider the following equation in aqueous solution: PO₃³⁻(aq) + SO₂(g) → PO₄³⁻(aq) + S²⁻(aq) What is the oxidation number on P in PO₄³⁻?

Answer 1

The oxidation number (state) of P in the PO₄^3- ion would be +5.

How do you get there:

You need to know the rules for assigning the oxidation number, and here two are important:

1. The sum of the oxidation states of all atoms forming the ion is the charge of the ion

2. O has an oxidation state of -2, except when in an peroxide, there it is -1

The phosphate ( PO₄^3- ) ion has no peroxide present. As such O has an oxidation state of -2.

Since we have 4 O atoms which contribute -2 each, the total charges from O are -8, overall we have a negative charge of -3. As such -8 + x = -3. x being P in PO₄^3- . Solving for x yields +5, the oxidation state of P in PO₄^3- .

I hope this helps.