Chlorine gas (8 mg/l as Cl2) was added to a drinking water

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Chlorine gas (8 mg/l as Cl₂) was added to a drinking water sample. If the free chlorine residual and pH was measured to be 2 mg/l (as Cl₂) and 7.5, respectively, what is the concentration of residual OCl^- ions in the water? Assume that the chlorine gas added to the water is completely converted to HOCl and OCl^-. Atomic weight of Cl: 35.5.
Given: OCl^- + H⁺ ⇄^K HOCl, K = 10^7.5

1. 1.408 × 10^–5 moles/l
2. 2.817 × 10^–5 moles/l
3. 5.634 × 10^–5 moles/l
4. 1.127 × 10^–4 moles/l

Correct Option: D

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