For a specific amino acid, the successive acid dissociation constants are strictly (for the group), — Ionic Equilibrium Chemistry Question
Question
For a specific amino acid, the successive acid dissociation constants are strictly $pK_{a1} = 2.18$ (for the $-COOH$ group), $pK_{a2} = 8.96$ (for the $-NH_3^+$ group), and $pK_{a3} = 10.53$ (for an additional basic side chain). At what precise pH will the molecule exist primarily as an electrically neutral zwitterion (the isoelectric point)?
Answer: B
💡 Solution & Explanation
The isoelectric point ($pI$) implies the net charge on the species must be exactly zero. Considering it as a triprotic acid, the fully neutral zwitterionic form is stabilized strictly between the second and third ionizations (where the basic side group and amino group are balanced). Thus, $pI = \frac{pK_{a2} + pK_{a3}}{2} = \frac{8.96 + 10.53}{2} = \frac{19.49}{2} = 9.74$.
💬Ask on WhatsApp →
Still have doubts about this question?
Send it to our AI chemistry tutor on WhatsApp — gets answered in minutes