Chemistry
posted by Melissa .
A 23.0mL sample of a diprotic acid is titrated with 0.260M KOH. If 55.5mL of base is required to reach the second equivalence point, what is the concentration of the acid? I'm not sure how to do this.

H2A + KOH ==> KHA + H2O
KHA + KOH ==> K2A + H2O
If 55.5 mL KOH were required to titrate to the second equivalence, then 55.5/2 mL were required to reach the first equivalence point.
(55.5/2)*0.260 = ?? mmoles KOH.
mmoles H2A = the same.
M H2A = mmoles/mL
You know mmoles H2A and mL H2A, solve for M 
7.2