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Which of the following compound doesn't contain peroxy oxygen?

  1. $\ce{CrO5}$
  2. $\ce{Cr2O3}$
  3. $\ce{Na2O2}$
  4. $\ce{BaO2}$

I think peroxy oxygen is peroxide, am I right?. I have read in this answer that there is no direct way to recognise peroxide oxygen, and I think that's the reason no one has taught me that in the school.

I am familiar with many peroxides like $\ce{Na2O2}$, $\ce{BaO2}$, etc. But I not able to recognise between 1 and 2.

Is there any way for this type of question to recognise peroxide oxygen? And if there is no way to determine then really I need to remember all, because we can't use X-ray crystallography in exam or while studying high school chemistry!


Answer given is 2. $\ce{Cr2O3}$

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  • $\begingroup$ try to draw strutural formula using highest valency for the element. It is pretty straighforward way to get the answer by elimination. $\endgroup$ – permeakra Oct 12 '14 at 6:59
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I have become familiar with many peroxide like Na2O2, BaO2, etc. But not able to recognise between 1 and 2. Is there any way for this type of question to recognise peroxide oxygen?

Yes. In the exam, if you do not know whether (1) or (2) contains peroxy, simply look at the charges of Chromium if all the oxygen is in the -2 oxidation state.

For $\ce{CrO5}$, if all the oxygen have a charge of -2, then Cr would have to have a charge of +10, which is impossible, therefore you know that some of the oxygen must have a charge of -1(peroxy). On the other hand, for $\ce{Cr2O3}$, Cr has a charge of +3, which is possible (Cr(III) salts are plenty)

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I'd like to offer an alternative method here, which is generic for all binary compounds (but may also work for ternery, quarternery, etc.).

Look at the highest oxidation state of the metal. Then divide the negative by the number of oxygens. This is how you find the average oxidation state of oxygen. If this is greater than -2, then there has to be a peroxyl oxygen involved. If it is -2, or lower, then there is no peroxyl involved. Additional remark: If the average oxidation state of oxygen is lower than -2, then the metal is not in its highest oxidation state.

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  • $\begingroup$ "If this is greater than -2, then there has to be a peroxyl oxygen involved." I'm probably missing something, but couldn't the oxygen be a superoxide instead? In a superoxide $\ce{O_2-}$, each $\ce{O}$ atom has an average oxidation state of $-0.5$, whereas in peroxide each oxygen has an oxidation state of $-1$ only... $\endgroup$ – Gaurang Tandon Mar 1 '18 at 14:42

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