WHO guidelines recommend that the amount of residual chlorine in drinking water should not be higher than 5 parts per million (ppm). The amount of chlorine can be determined by back titration with potassium iodide:
Cl₂(aq) + 2 I⁻(aq) → 2 Cl⁻(aq) + I₂(aq)
The iodine formed as a result of the reaction above is then titrated with a solution of sodium thiosulfate:
2 Na₂S₂O₃(aq) + I₂(aq) → 2 NaI(aq) + Na₂S₄O₆(aq)
A 350.0 mL water sample was added with excess solid sodium iodide and the resulting solution was then titrated against a 2.25×10⁻³ M sodium thiosulphate solution. The solution required 14.5 mL of Na₂S₂O₃ to reach the equivalence point. How many grams of chlorine are there in the water sample?

• A. 5.80×10⁻⁴ g chlorine
• B. 1.16×10⁻³ g chlorine
• C. 2.32×10⁻³ g chlorine
• D. 1.74×10⁻³ g chlorine

Answer: (B)