Sodium hypochlorite is a common ingredient in household bleach. What is the pH of this bleach if it contains 5% NaOCl by mass? (pK_a of HOCl = 7.46)

Question

Quizplus · Accepted Answer

Sodium hypochlorite (NaOCl) is a salt of hypochlorous acid (HOCl), a weak acid. When HOCl dissociates in water, it forms H+ ions and OCl- ions. The equilibrium constant for this dissociation is given by the pKa value of HOCl, which is 7.46.

HOCl ⇌ H+ + OCl-

When NaOCl is added to water, it dissociates completely to form Na+ and OCl- ions. Since OCl- is the conjugate base of HOCl, it can react with water to form HOCl and OH- ions. The equilibrium constant for this reaction is given by the pKb value of OCl-, which is 6.54.

OCl- + H2O ⇌ HOCl + OH-

Therefore, the pH of the bleach solution will depend on the balance between the concentrations of HOCl and OCl- ions. If the concentration of OCl- ions is higher, the solution will be more basic (pH > 7) due to the formation of OH- ions from the reaction above.

Since the bleach contains 5% NaOCl by mass, we can assume that the concentration of OCl- ions is 5% as well. Therefore, the concentration of HOCl ions will be (100% – 5%) = 95%.

pH = pKa + log ([OCl-]/[HOCl])

pH = 7.46 + log (0.05/0.95)

pH = 11.13

Therefore, the pH of the bleach solution is approximately 11, and the best choice is B.

Sodium Hypochlorite Is a Common Ingredient in Household Bleach