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Question

Hypochlorous acid, HClO, has a pKa of 7.54. What are [H3O+], pH, [ClO-], and [HClO] in 0.131 M HClO?

Answer 1

It is a realtively long calculus but I will try to do it the easiest to understand.

1) The reaction of dissociation observed is:

HClO + H2O <<--------->> H30(+) + ClO(-)

2) We know the concetration of the HClO solution: 0.131 M

3) As we know the pKa = 7.54 and the definition :

pKa = -log Ka

then: Ka = 10^-pKa = 10^-7.54= 2.88 x 10^-8

4) You surely know that Ka is defined as follows:

Ka = [H3O+] [ClO-] / [HClO]

5) According to above equation and that solution is 0.131 M:

2.88 x 10^-8 = [H3O+] [ClO-] / 0.131M

we assume that concentration of both H3O+ and ClO- is the same according to the equation above:

6) 2.88 x 10^-8 = [x]² / 0.131 M being x=unknown concentration of both species

We solve for x:

x = 6.14 x 10^-5 M

7) We resume.

[HClO] = 0.131 M
[H3O+] = 6.14 x 10^-5M
[ClO] = 6.14 x 10^-5M

That´s it!

Good luck!