Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this redox change taking place in water. 

The skeleton equation is
           
Let us balance the above equation by ion electron method. 
1. Write the oxidation and reduction half-reaction by observing the change in oxidation number and writing these separately
Oxidation half-reaction:

2. Balancing the oxidation half reaction: Reduction half reaction:

(i) Add 2 electrons towards R.H.S. to balance the change on S.
                 
(ii) Balance charge by adding four H⁺ towards R.H.S.
                     
(iii) Balance O atoms by adding two H₂O molecules towards L.H.S.

  (Balanced oxidation half- reaction)
3. Balancing the reduction half reaction:
   (i) Balance Cl atoms by multiplying Cl^- by 2,
             
   (ii) Add e^- towards L.H.S.  to balance the charges
          
4. Adding balanced oxidation half reaction and balanced reduced half reaction.
                   
This is the balanced redox equation.