Multiple choice

Chlorine gas used for disinfection combines with water to form hypochlorous acid (HOCl). HOCl ionizes to form hypochlorite (OCl-) in a reversible reaction: HOCI $\Leftrightarrow$ H+ + OCl- (k= 2.7 × 10-8 at 20 oC), the equilibrium of which is governed by pH. The sum of HOCI and OCl- is known as free chlorine residual and HOCI is the more effective disinfectant. The 90% fraction of HOCl in the free chlorine residual is available at a pH value

  1. 4.8

  2. 6.6

  3. 7.5

  4. 9.4
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The fraction of HOCl is given by 1 / (1 + K/H+). For 90% (0.9) fraction, 0.9 = 1 / (1 + 2.7e-8 / H+). Solving for H+ gives H+ = 2.43e-7, and pH = -log(H+) = 6.61.

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