Question

An unknown triprotic acid (H3A) is titrated with NaOH. After the titration Ka1 is determined to be 0.0053 and Ka2 is determined to be 1.5e-07. The pH at the second equivalence point of the titration is measured as 8.469. What is Ka3 for this triprotic acid?

Answer 1

Answer:

Ka3 for the triprotic acid is 7.69*10^-11

Explanation:

Step 1: Data given

Ka1 = 0.0053

Ka2 = 1.5 * 10^-7

pH at the second equivalence point = 8.469

Step 2: Calculate Ka3

pKa = -log (Ka2) = 6.824

The pH at the second equivalence point (8.469) will be the average of pKa2  and pKa3. So,

8.469 = (6.824 + pKa3) / 2

pKa3 = 10.114

Ka3 = 10^-10.114 = 7.69*10^-11

Ka3 for the triprotic acid is 7.69*10^-11