[mtndew2000]

I have 60 ml solution of household bleach (5 percent sodium hypochlorite; 3 ml) at pH 12.6. How many milliliters of 10 percent HCl at pH 1.0 should be added to reduce the pH of the mixture solution to a pH of exactly 5.0? I know in advance that chlorine gas will not be liberated from the solution until the pH approaches 4. I also know that at pH 5 the species of chlorite ion will be virtually 100 percent HOCl, hypochlorous acid. Is there a better way to answer this question than by using a Hanna pH meter in the lab?

[Borek]

using some standard methods (stoichiometry, pH of weak acids) you can estimate voume of acid necessary, but the resulting solution will have so high ionic strength that it is not possible to calculate the result exactly.

[Dingle]

1. from the acid dissociation constant, Ka, of Hypochlorous acid, the Henderson-Hasselbalch equation, and moleculear weight of HOCl and ClO-, work out how many moles of ClO- you have in the 60 mL

2. then you need to add enough moles of H+ ions (from HCl - it dissociates into H+ ions completely) to quench all the ClO- and extra to give pH = 5 = -log[H+] = log(1/[H+])

Hope you can figure it out