How to calculate Ka and Kb for KHP based on pH?

Question

I need help on how to:

Calculate the acid dissociation constant Ka for weak acid KHP based on A) the initial pH of the solution at 0 ML of NaOH added and B) the pH at 1/2 the volume of the endpoint. The initial pH of the solution at 0 mL equals 4.1 and the pH at 1/2 the volume of the endpoint is 5.14. Then I have to calculate the base dissociation constant Kb for the conjugate base of KHP, which is phthalate. This is determined from the Ka value and Kw usint the relationship that Ka*Kb=Kw calculate the Kb from both Ka values above.

I have no idea how to do this... anything would be helpful! Thanks!

Answer 1

@ a 1/2 titration of KHP:

50% KHP --> H+  & 50% P-2

so regardless of the initial concentrations :

K = [H+] [P-2]  / [KHP]

K = [H+] [50%]  / [50%]

===================

K = H

so:  PK = PH  = 5.14

do a 10 ^-x with the 5.14  &

K = H+ = 7.2e-6

your answer is Ka = 7.2 e-6

===============================

Kb = K water / Ka = 1e-14 / 7.2e-6 = 1.4 e-9

======================================

to get the Ka from the initial pH, I would need the molarity of the KHP

you can e-mail me by clicking on my avatar