Question: Compound X has two ionizable groups, with one pK_a of 3.8 and a second pK_a between 7 and 10. A biochemist has 10 ml of a 1.0 M solution of compound X, at a pH of 7.9. She adds 10 ml of 1.0 M HCl, which changes the pH to 3.2. What is the pK_a of the second ionizable group?
Second pK_a=?
Answer: 8.5
8 ml of the HCl was used to titrate the pK_a 3.8 group; so 2 ml was enough to titrate the rest of the high pK_a group. The upper pK_a group must have begun the experiment as 80% acid form, 20% base form.