Chlorine in hypochlorous acid for water disinfection

Steps:

• Chlorine gas (Cl₂) is added to water, reacting to form hypochlorous acid (HOCl), the primary bactericide.
• HOCl dissociates slightly to hypochlorite ion (OCl⁻), but HOCl is the active species.
• To find Cl oxidation state in HOCl, assign known values: H = +1, O = -2.
• Sum: +1 (H) + x (Cl) - 2 (O) = 0, so x = +1.

Why B is correct:

• In HOCl, the neutral molecule requires Cl at +1 to balance H (+1) and O (-2), per oxidation state rules.

Why the others are wrong:

• A (-1): Matches chloride ion (Cl⁻), a reduced, non-disinfecting form.
• C (+3): Occurs in chlorite (ClO₂⁻), not used for bacterial killing in standard treatment.
• D (+5): Found in chlorate (ClO₃⁻), an oxidized byproduct, not the active disinfectant.

Final answer: B