Question

Given that 20 ml of a weak acid (HA) was titrated with NaOH and the titration curve of pH vs titrant volume in ml is given, find the initial concentration of the acid?

The problem gives the full titration curve starting at pH=2.2 at 0 ml added and with the equivalance pt at pH=7 at 18 ml added.

Answer 1

I'm at a distinct disadvantage since I can't see the titration curve but I think this is what you do.
The equivalence point is 7 (which I really don't understand since weak acids and strong bases are supposed to be somewhere in the 8.3 range for the eq pt but this is probably just a made up problem). However, with the eq pt at 7.0 and 18 mL., take 1/2 of the 18 and look up on the curve for the pH at 9.0 mL. That will give you the pKa value. Convert that to Ka by pKa = -log Ka.
Then convert pH 2.2 to (H^+) and plug that into the Ka equation of
Ka = (H^+)(A^-)/[(HA)-(H^+)] and solve for HA.