I have no idea how to do this can someone please help
1 Answer

The Ka, the acid dissociation constant, is define for some acid HX as follows:
Ka = [H+][X] / [HX]
So, for hypochlorous acid, it is as follows:
Ka = [H+][OCl] / [HOCl]
So, how do we know any of these numbers? We know the pH, and, since pH = log[H+], we can find [H+] concentration. Since dissociation results in equal amounts of hydrogen ions and hypochlorite, the [H+] we calculate will be equal to the value of [OCl]
pH = – log[H+]
4.23 = log[H+]
[H+] = 5.9*10^5 M
So, now we know [H+] and [OCl]. How do we know [HOCl]? Well, we started out with .1 M. Technically, 5.9*10^5 M dissociates, so we could subtract that from 0.1 to get our term, but it won’t make an appreciable difference, so we can just say that [HOCl] = 0.1
Ka = (5.9*10^5 M)(5.9*10^5 M) / .1
Ka = 3.47 * 10^8
Now, we can find pKa.
pKa = – log Ka
pKa = – log (3.47 * 10^8)
pKa = 7.46