Aqueous Equilibrium: Ka of an Unknown Acid via Titration

[Svennerson]

Homework Statement

We're given an unknown acid, and dissolve an unknown mass of the acid in an unknown amount of water. We then titrated the solution with an unknown molarity solution of NaOH. After titrating in 10.0 mL of NaOH, we measure a pH of 5. After 34.68 mL of NaOH, we realize we forgot to add phenolphthaelin. Adding it in turns the solution deep pink. Adding in another 2.62 mL of the unknown acid solution turns the solution colorless again. Find the Ka of the acid.

We are absolutely stumped on this one, please help.

 

[Borek]

I can be missing something, but I don't see how to solve without knowing the initial volume of the acid solution.

 

[Svennerson]

I'm an AP Chem tutor, and this stumps me for the same reason.