Since the equivalence point for this titration curve lies above pH = 7 (around pH
= 9), and the titration was done using an NaOH solution as the titrant, the
original solution contained a weak acid.
Determining pKa (pKb)
First, a warning: NEVER try to figure out the pKa or pKb of a strong acid or base
—they react completely.
Now, figuring out the pKa of a weak unknown acid (or pKb for a weak unknown
base) requires finding a second location on the titration curve: the half-
equivalence point. The half-equivalence point, as its name suggests, is the point
at which enough titrant has been added to neutralize exactly one-half of the
original unknown acid or base. You can identify this point by locating the
equivalence point and then backtracking halfway to zero along the x-axis.