Question

What is the oxidation number of carbon in ${H_2}C{O_3}$
A.$+ 1$
B.$+ 2$
C.$+ 3$
D.$+ 4$

Answer 1

All ionic compounds like carbonic acid given by the formula ${H_2}C{O_3}$ are neutral in nature and therefore the charges contributed by each ion are balanced by counter ions in such a manner that the net oxidation number also become neutral like the charges.

Complete answer:
Carbonic acid is a weak mineral acid that dissociates in water to give $2{H^ + }$ cations and one $C{O_3}^{2 - }$ anion.
Oxidation number is a hypothetical number assigned to each atom in a molecule to measure the number of electrons contributed by that atom in forming the bonds of the molecule.
Since carbonic acid is a neutral compound (has no net charge on the molecule), we take the net oxidation number to be equal to zero.
In order to find out the oxidation number of carbon, let its oxidation number be $x$.
Assign oxidation numbers to the remaining atoms:
$H \to + 1$
$O \to - 2$
Now multiply the oxidation number of each atom by the total number of atoms present in the molecule:
$2H \to + 2$
$3O \to - 2 \times 3 = - 6$
We know that the overall oxidation number is zero. Hence the total oxidation numbers from each element can be added and equated to zero in order to find out the value of $x$:
$+ 2 + x - 6 = 0$
The above equation is a simple linear equation in one variable and can be solved to find out the value of $x$.
$x = 6 - 2 = ( + 4)$
Thus the correct option is option (d) and the value of oxidation number of carbon atoms in ${H_2}C{O_3}$ is $+ 4$.

Note:
The oxidation number should not be confused with valency as an atom is capable of showing variable oxidation states in different types of molecules. The oxidation number can be higher or lower than the valency of the atom and is never fixed.