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 is a basic solution with a pH value greater than 7. In this case, we can calculate the pH using the equilibrium reaction: 
NaOCl + H2O <=> HOCl + Na+ + OH-

Using the given pKa value of HOCl, we can calculate the equilibrium constant (Ka) = 10^-7.46 = 3.0x10^-8. Then we can use the equation:

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

Since the bleach contains 5% NaOCl by mass, we can assume that the solution is mostly water and can approximate [OCl-] to be 0.05 M. This gives:

pH = 7.46 + log(0.05/[HOCl])

We can solve for [HOCl] using the fact that the bleach is around 5% NaOCl by mass. We assume the density of bleach is about the same as water (1 g/mL) so that 5% NaOCl corresponds to 50 g/L or 0.763 M.

[OCl-] + [HOCl] = 0.763 M
0.05 M + [HOCl] = 0.763 M
[HOCl] = 0.713 M

Substituting this into our pH equation:

pH = 7.46 + log(0.05/0.713) = 10.93

This is closest to choice B, pH 11.

Sodium Hypochlorite Is a Common Ingredient in Household Bleach