Question

how to find oxidation no of cao2 ?
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Answer 1

if we the oxidation no. of calcium(ca)=?
let consider the oxidation no of calcium (ca)=X
we know that
oxidation no of oxygen(O)=-2
caO2
---
X+(2x-2)=0
X-4=0
X=+4
So oxidation no of calcium (ca)=+4

Answer 2

That being said, this is tricky one, CaO2 is also konown as calcium peroxide , and you may recall that peroxide have their own rule:

Oxygen has oxidation number-1 in peroxide instead of its usual -2.

This allows calcium to have its normal oxidation number of +2.

Thus: oxygen, -1; calcium, +2.

Barium suicide BaO2 is also a peroxide,btw.