Explanation:
The HCO₃ in NaHCO₃ has an oxidation state of -1. We know this because of the charge on HCO₃ and also by the fact that Na is a group 1 metal that always has a +1 oxidation state.
The sum of the oxidation states of the components of HCO₃ has to be equal to -1
⇒ H + C + 3 O = - 1
A rule of redox is that Oxygen always has a - 2 oxidation state and H usually has a + 1 oxidation state
⇒ 1 + C + 3(-2) = - 1 [make C the subject of the equation and simplify]
⇒ C = -1 - 1 + 6
C = + 4
