Thank you Leah, recently ran into your youtube and wanted to thank you for such informative videos. Although I am not sitting the MCAT, I am sitting the GAMSAT (similar, just an Australian version) and these videos are highly relevant.
The rules state that the sum of all oxidation numbers in a neutral compound should be zero (see #3 on my list). Since CO2 is a neutral compound, the total of all its oxidation numbers should be zero. Oxygen, by itself, typically has an oxidation number of -2. Other atoms in a compound with oxygen will have to adapt their oxidation state to fit the oxygen. That’s why carbon in CO2 ends up with an oxidation number of +4.
Could you be more specific about what you don't understand? I address this topic at 8:45 in the video. The peroxy ion is composed of two oxygens single bound to each other, where each oxygen atom has an oxidation state of -1. In H2O2, the hydrogen atoms would each have an oxidation state of +1.
