Final answer:
The oxidation number of carbon in oxalic acid (H2C2O4) is calculated by assigning -2 to oxygen and +1 to hydrogen, resulting in an expected +3 oxidation state per carbon atom when balanced. However, this result does not match any of the given answer choices, indicating a possible mistake or the need for further context.
Step-by-step explanation:
To determine the oxidation number of carbon (C) in oxalic acid (H2C2O4), we can follow standard rules for assigning oxidation numbers. By rule, we assign oxygen (O) an oxidation number of -2, and hydrogen (H) an oxidation number of +1. The molecule is neutral, so the overall charge is zero, meaning the sum of all the oxidation numbers must equal zero.
Oxalic acid has two hydrogen atoms, four oxygen atoms, and two carbon atoms. Using the assigned oxidation numbers for oxygen and hydrogen (rule 4 and 5), the total charge for hydrogen is (+1)*2 = +2 and for oxygen is (-2)*4 = -8. For the two carbon atoms to balance this out to zero, they must have a combined oxidation number of +6 (since +2 - 8 + 6 = 0).
Since there are two carbon atoms, the oxidation number for each carbon atom must be +3. However, in this structure, both carbon atoms are equivalent, and they are both connected to oxygen atoms in the same way, meaning they should have the same oxidation number. Thus, their individual oxidation number is taken to be +3. However, considering this might be a misinterpretation of the question which could be asking for the oxidation state of C as it appears in a standard organic functional group, for a carboxylic acid group, such as that in oxalic acid, the oxidation number of carbon is generally <+3>. This is because each carbon is double-bonded to one oxygen and single-bonded to an OH group, which would suggest an oxidation number of +3. But upon further analysis for oxalic acid, where both carbons are in identical environments being bonded to two oxygens (one in an OH group and another in a C=O double bond), it would be more accurate to consider both carbons sharing the oxidation states equally resulting in an oxidation state of +3 for each carbon, which is an unconventional approach as it does not match any of the given options.
However, this unconventional result might not be accepted in typical chemistry assignments. Hence, it's important to cross verify with the standard methodologies or academic resources or the context of the question, which suggests the oxidation number of a carbon atom in a carboxylic group is +3, but if considering the sum is to be split evenly between two identical carbons, each would have an oxidation number of +3.
As none of the choices provided (+2, -2, +4, -4) match the calculated oxidation number, there might have been a mistake in the question or in the interpretation here. Without additional context, we cannot provide a definitive answer matching the provided options. Please review your question or provide additional context for a more precise answer.