Question

A scientist finds a new acid which she knows is diprotic. She only has a solution which is 0.1000
in this diprotic acid H2Acid and she only has 50.00mL of it. She decides she must find out what the pKa1
and pKa2 is for the diprotic acid. She has a pH meter which has been calibrated and a 0.1000M NaOH
solution. Making only two pH measurements, how would she estimate pKa1 and pKa2. Please be
specific about your answer. Which measurements would you need, how would you make such a solute
and what equations would you used for this purpose?

Answer 1

The weak diprotic acid is titrated with strong base NaOH.

The equations corresponding to two equivalence points are given below.

At first half equivalence point

At second half equivalence point

A graph of pH vs volume of NaOH added is shown below.

Thus we start with 10.00 mL of 0.1000 M weak diprotic acid H2A and place it in the conical flask. We fill the burette with 0.1000 M NaOH solution. When we add 5 mL of 0.1000 M NaOH solution to the conical flask, first half equivalence point will be attained. At this point, we measure the pH of solution which will correspond to . Now we add additional 10 mL of 0.1000 M NaOH. Total volume of NaOH added will be 15 mL. Second half equivalence point will be attained. At this point, we measure the pH of solution which will correspond to
.