The graph represents the titration of an amino acid with NaOH solution. A. At what pH is the average net charge 1/2? pH = B. Where does the amino acid have a net charge of −1? above pH 9.69 at pH = 9.69 at pH = 6.01 at pH = 2.34 below pH 2.34 C. At what point has enough base been added to react with 1/2 of the NH3+ groups? 1/2 of NH3+ reacted = equivalents OH- when 1/2 of NH3+ has reacted, the pH = D. At which pH value does the amino group of amino acid have the best buffering capacity? 1.53 9.69 9.00 10.5 3.00 6.01 E. What is the pI (isoelectric point)? pI =
The Correct Answer and Explanation is:
Let’s break this down step by step based on the typical behavior of amino acids during titration with NaOH:
A. At what pH is the average net charge 1/2?
This is often observed at the pKa of the amino acid’s NH₃⁺ group (the protonated amine group). When the net charge is 1/2, it’s likely that the amino acid is partially in the form of NH₃⁺ and partially as NH₂. This would occur at the pKa of the NH₃⁺ group, which is usually around pH = 9.69.
B. Where does the amino acid have a net charge of −1?
The amino acid will have a net charge of −1 when the carboxyl group (COOH) has lost its proton, becoming COO⁻, and the amino group (NH₂) is in its neutral form (NH₂). Based on the pKa values, this typically occurs at pH = 6.01, where the amino acid is in a zwitterionic form, but the NH₂ group is deprotonated, and the carboxyl group is negatively charged.
C. At what point has enough base been added to react with 1/2 of the NH₃⁺ groups?
When 1/2 of the NH₃⁺ groups have reacted, we are essentially at the pKa of the amino group. At this point, the amino acid behaves as a buffer. This can occur at pH = 9.69, where the pH is approximately equal to the pKa of the NH₃⁺ group.
D. At which pH value does the amino group of the amino acid have the best buffering capacity?
The buffering capacity of an amino acid is best at the pKa of its functional group. Since the amino group (NH₃⁺ → NH₂) has a pKa around 9.69, this is where the best buffering capacity occurs for the amino group. Hence, the correct answer would be pH = 9.69.
E. What is the pI (isoelectric point)?
The pI (isoelectric point) is the pH at which the amino acid has no net charge. This is the average of the pKa values for the amino acid’s NH₃⁺ and COOH groups. If the pKa values are 9.69 for NH₃⁺ and 2.34 for the carboxyl group, the pI can be calculated as:pI=(pKa1+pKa2)2=(9.69+2.34)2=6.015pI = \frac{(pKa1 + pKa2)}{2} = \frac{(9.69 + 2.34)}{2} = 6.015pI=2(pKa1+pKa2)=2(9.69+2.34)=6.015
So, the pI = 6.01.
Summary Answers:
- A. pH = 9.69
- B. pH = 6.01
- C. pH = 9.69
- D. pH = 9.69
- E. pI = 6.01
These points reflect the typical amino acid titration curve where the pKa values guide the behavior of the amino and carboxyl groups as they titrate.
