What is the oxidation number of O in PO4 3-?

Answer 1

PO4 3-

The O is almost always -2, so, 4 x -2 = -8

the oxidation number of P + -8 = -3

Therefore the oxidation number of P is +5

Answer 2

The oxidation number of the oxygen in PO₄^(-3) is -2. Phosphorous can have oxidation states of -3, +3, +5. However, in this anion, phosphorous has a oxidation state of +5. Outside of peroxides and certain compounds, oxygen is usually has an oxidation state of -2. If you want to check this, set up the following equation:

1(oxidation state of Phosphorous) + 4( oxidation state of oxygen) = charge of anion

1( +5) + 4(-2) = -3

-3 = -3

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