How to Find the Ka of an Acid from pH and Titration Data?

[Jade_punk181]

Homework Statement

A solid acid is dissolved in enough water to make 200 mL of a solution. 40.0 mL of the solution is titrated to a phenolphtalein end point with an NaOH solution. The neutralized solution and the remainder of the acid solution are then mixed and the pH of the resulting solution is found to be 3.35. Find the Ka of the acid.by trying to figure out this problem, I assumed that the following formula would come in useful:

((concentration of HA)/(concentration of A-))=volume of acid/volume of base=((concentration of H+)/(Ka))I have attempted this problem, but the equations I had weren't fit for this problem.

 

[Borek]

After mixing you have a solution containing acid and its conjugate base in known proportions.

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