One of the most important industrial applications of redox titrations is evaluating the chlorination of public water supplies. One approach for determining the total residual chlorine in treated water uses the oxidizing power of chlorine to oxidize I- to iodine. If this reaction is carried out in the presence of excess iodide the soluble triiodide ion, I3-, is formed. The amount of I3- is determined by back titration with thiosulfate, S₂O3²-, which is oxidised to the tetrathionate anion, S406²-. (i)Write a balanced net ionic equation for the reaction between Chlorine and iodide in an excess of iodide. (ii)Write a balanced equation for the reaction between thiosulfate and triiodide. The world Health Organisation taste guideline for chlorinated water is 5 mg.L-¹. A sample of Canberra tap water was tested for its residual chlorine content using the redox titration process described above. A 500 mL sample of water was treated with 5 g of sodium iodide to convert the residual chlorine to chloride. The triiodide was then determined by taking 25 mL a liquots of the water and titrating it with 0.1409 M sodium thiosulfate. The results of the titrations were as follows:

(iii) Determine the average titre. (iv)What is the triiodide concentration? (v)What was the concentration of the chlorine in the initial water sample? (vi)Does the water sample meet the WHO guideline?