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

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)

Quizplus · Accepted Answer

First, we need to write the balanced equation for the reaction that occurs when NaOCl dissolves in water:
NaOCl + H2O ↔ HOCl + Na+ + OH-
Since bleach contains 5% NaOCl by mass, we can assume that the solution is approximately 5% NaOCl by weight. To find the concentration of NaOCl in moles per liter (M), we need to divide the percent mass by the molecular weight of NaOCl:
5% NaOCl = 5 g NaOCl / 100 g solution
Molecular weight of NaOCl = 74.44 g/mol
5 g NaOCl / 74.44 g/mol = 0.067 M NaOCl
Next, we need to determine the pH of the solution. We know that when NaOCl dissolves, it forms HOCl and OH-. The pH of the solution depends on the relative concentrations of these two species. To calculate the pH, we need to use the pKa of HOCl:
pKa = 7.46
pH = pKa + log([HOCl]/[OCl-])
We can assume that the concentration of Na+ is negligible and does not affect the pH. Therefore,
[OCl-] = 0.067 M
[HOCl] = Kw /[OH-] = 1.0 x 10^-14 / [OH-]
Substituting these values into the equation for pH,
pH = 7.46 + log([1.0 x 10^-14]/[0.067 x OH-])
pH = 7.5 + log(1.5 x 10^-13/[OH-])
pH = 7.5 - 12.8 + log(1/[OH-])
pH = -5.3 + log(1/[OH-])
pH = -5.3 - log([OH-])
pH = -5.3 - log(5.9 x 10^13)
pH = 11.3
Therefore, the pH of bleach containing 5% NaOCl by mass is approximately 11.

Sodium Hypochlorite Is a Common Ingredient in Household Bleach