Question

Which of the following is NOT true regarding the value for pKa based on a titration curve using a pH electrode to determine the equivalence point?

A.pKa = pH at 1/2 the volume of the equivalence point
B.pKa for polyprotic acid = pH at 1/2 the volume of the equivalence point and represents the average response of all the acidic protons (i.e. different pKa values)
C. pKa = pH at the equivalence point
D. pKa = pH at the first inflection point (i.e. point where slope changes sign) on the titration curve

Answer 1

Henderson-Hesselbalach equation:

pH = pKa + log { [salt] / [acid] }

At the one-half equivalence point, you have equal moles of weak acid and base.

So,

So, log { [salt] / [acid] } = log 1 = 0

Whatever buffer calculation you use will yield pH = pKa.

At the equivalence point, you will have converted all of your weak acid to its conjugate base or vice versa.

Therefore, at the half-equivalence point, the pH is equal to the pKa.

Since, the statement C is not true.

So, Answer is (C)