Determine oxidation state for C in CO2 and CH4.

• A. CO2: 0; CH4: 0
• B. CO2: +2; CH4: -2
• C. CO2: +4; CH4: -4
• D. CO2: -4; CH4: +4

Answer: (C)

Oxidation state is a bookkeeping method where charges are assigned to atoms to make the sum of a neutral molecule zero. In most compounds, oxygen is -2 and hydrogen is +1. For a neutral molecule, the total must add up to zero.

In carbon dioxide, there are two oxygens, contributing -2 each for a total of -4. To balance to zero, the carbon must be +4. In methane, there are four hydrogens, contributing +1 each for a total of +4. To balance to zero, the carbon must be -4. So carbon is +4 in CO2 and -4 in CH4.

These results agree with the standard rules and sums; choosing other values would make the total charge inconsistent with a neutral molecule.