Another oxidation number of a compound....please help?

Question

I will be tested on this in 2 weeks so I am practicing.
For CO2 ,would the carbon be at either a +4 or a +2 because the oxygen is -2 or -1 or would the oxygen be a -4 or -2 because it is two oxygen molecules.

For Na2CO3 , the sodium is 1 x 2 = +2, the oxygen is 3 x -2 = -6 ,so the carbon would have to be +4 to balance it out?
THANKS

Answer 1

C is +4, because O is -2. 2(-2) = -4, which balances C = +4. O is always -2, except in peroxides like H2O2 or Na2)2, where it is -1.

Na2CO3. C = +4. You're exactly right.