How do you balance redox equations by oxidation number method?
1 Answer
You follow a series of steps in order.
For example, balance the equation
HNO₃ + H₃AsO₃(aq) → NO(g) + H₃AsO₄(aq) + H₂O(l)

Identify the oxidation number of every atom.
Left hand side: H= +1; N= +5; O = 2; As = +3
Right hand side: N = +2; O = 2; H = +1; As = +5 
Determine the change in oxidation number for each atom that changes.
N: +5 → +2; Change = 3
As: +3 → +5; Change = +2 
Make the total increase in oxidation number equal to the total decrease in oxidation number.
We need 2 atoms of N for every 3 atoms of As. This gives us total changes of 6 and +6. 
Place these numbers as coefficients in front of the formulas containing those atoms.
2HNO₃ + 3H₃AsO₃(aq) → 2NO(g) + 3H₃AsO₄(aq) + H₂O(l) 
Balance all remaining atoms other than H and O.
Done. 
Balance O.
Done. 
Balance H.
Done.
Now try to balance the equations in the link below (answers included).