Calculate pKa and Ka

Question

In a lab we collected the equivalence point pH and the pH of the half-titrated solution and are asked to calculate the pKa and Ka. 

Answer 1

Elizabeth,

 

If it's not too late for the lab:

 

The acid HA dissociates in solution as HA = H⁺ + A^-.

 

Ka = [H+][A-] / [HA]

 

and

 

pKa = pH + log{ [A-] / [HA]}

 

When you are halfway to the equivalence point, you have neutralized half the HA and turned it into and an equal amount of A- (and H+). So, [HA] = [A-], [A-]/[HA] = 1, log{ 1 } = 0, and pKa = pH(at the halfway point).

 

Michael.