Chemistry: Principles and Reactions
Chemistry: Principles and Reactions
8th Edition
ISBN: 9781305079373
Author: William L. Masterton, Cecile N. Hurley
Publisher: Cengage Learning
Question
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Chapter 23, Problem 30QAP
Interpretation Introduction

(a)

Interpretation:

The pH at which the zwitter ion [Z] and the cationic form [C+] of the amino acid alanine become equal needs to be calculated.

Concept introduction:

  1. Amino acids are organic compounds composed of C, H, N and O. The two main functional groups include the amino −NH2 and carboxyl −COOH group in addition to a side chain each with a characteristic pKa value.
  2. Isoelectric point (pI) is the pH at which the net charge on the amino acid is zero. For amino acid with one −COOH and one −NH2, the pI is given as:

pI = pKa1+pKa22 ------(1)

Expert Solution
Check Mark

Answer to Problem 30QAP

pH = 2.29

Explanation of Solution

The given amino acid is alanine which has the following structure:

Chemistry: Principles and Reactions, Chapter 23, Problem 30QAP , additional homework tip  1

The structures of the respective cationic form (C+), zwitter ion (Z) and the anionic form (A-) are:

Chemistry: Principles and Reactions, Chapter 23, Problem 30QAP , additional homework tip  2

The acid-base equilibria for alanine involving the cation and the zwitter ion forms is:

C+(aq) H+(aq) + Z(aq)Ka1=[H+][Z][C+] When [Z] = [C+]Ka1 = [H+]i.e.  [H+]=5.1×103 MpH = -log[H+]=log[5.1×103]=2.29

Interpretation Introduction

(b)

Interpretation:

The pH at which the zwitter ion [Z] and the anionic form [A-] of the amino acid alanine become equal needs to be calculated

Concept introduction:

  1. Amino acids are organic compounds composed of C, H, N and O. The two main functional groups include the amino −NH2 and carboxyl −COOH group in addition to a side chain each with a characteristic pKa value.
  2. Isoelectric point (pI) is the pH at which the net charge on the amino acid is zero. For amino acid with one −COOH and one −NH2, the pI is given as:

pI = pKa1+pKa22 ------(1)

Expert Solution
Check Mark

Answer to Problem 30QAP

pH = 9.74

Explanation of Solution

The given amino acid is alanine which has the following structure:

Chemistry: Principles and Reactions, Chapter 23, Problem 30QAP , additional homework tip  3

The structures of the respective cationic form (C+), zwitter ion (Z) and the anionic form (A-) are:

Chemistry: Principles and Reactions, Chapter 23, Problem 30QAP , additional homework tip  4

The acid-base equilibria for alanine involving the anion and the zwitter ion forms is:

Z(aq) H+(aq) + A(aq)Ka2=[H+][A][Z] When [Z] = [A]Ka2 = [H+]i.e.  [H+]=1.8×1010 MpH = -log[H+]=log[1.8×1010]=9.74

Interpretation Introduction

(c)

Interpretation:

The pH at the isoelectric point needs to be calculated

Concept introduction:

  1. Amino acids are organic compounds composed of C, H, N and O. The two main functional groups include the amino −NH2 and carboxyl −COOH group in addition to a side chain each with a characteristic pKa value.
  2. Isoelectric point (pI) is the pH at which the net charge on the amino acid is zero. For amino acid with one −COOH and one −NH2, the pI is given as:

pI = pKa1+pKa22 ------(1)

Expert Solution
Check Mark

Answer to Problem 30QAP

pH = 6.02

Explanation of Solution

For alanine at the isoelectric point:

pI =pH =  pKa1+pKa22 pKa1 = -log[Ka1] =log[5.1×103]=2.29 pKa2 = -log[Ka2] =log[1.8×1010]=9.74pH = 2.29 + 9.742=6.02

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