How to do a titration

Question

A student is doing a titration experiment with KOH and HBr. A solution of 0.20 M KOH and 0.10 M HBr are prepared. A pH electrode is used to obtain the data plotted in the titration curve shown below.

a.)At the equivalence point how many moles of titrant have been added?

b.)What is the Ka and pKa of the acid? How do you use the titration curve to determine these values?

Answer 1

The equivalence point corresponds to the middle of the vertical part of the graph which is at 40 mL of titrant. As the titrant is 0.20 M, the # moles added is

0.20 moles/L / 1000 mL /L x 40 mL = 0.008 moles.

At the halfway point of the titration, the acid and its salt are in equal amounts, constituting a buffer solution. According to the Henderson-Hasselbalch equation, pH = pKa + log [salt] / [acid]. [salt] / [acid] =1, so its log is zero and the pKa = pH. The estimated pH at 20 mL is about 1.7, so pKa is also 1.7.

As pKa = -logKa, take the antilog of -1.7 to get 2.0 x 10^-2