find the oxidation state of a carbon in methane, carbon monoxide, carbon dioxide, CH2O, acetic acid, carbonic acid, methanol, and benzene (C6H6)

Answer 1

Rules for oxidation state.
http://www.chemteam.info/Redox/Redox.html

C in CH4,
H is +1 and there are 4 so total charge on 4 H is +4. Since the molecule is zero, C must be -4

C in benzene, C6H6
H is +1. 6*+1 = +6.
Since benzene is 0 charge, then 6 C must be -6 and EACH C must be -1 each.