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The isoelectric point of an amino acid is the pH at which the molecule has no net charge. For glycine, that point would be the pH at which virtually all glycine molecules are in the form + H 3 NCH 2 CO 2 − . This form of glycine is amphoteric since it can act as both an acid and a base. If we assume that the principal equilibrium at the isoelectric point has the best acid reacting with the best base present, then the reaction is 2 + H 3 NCH 2 CO 2 - ⇌ H 2 NCH 2 CO 2 - + + H 2 NCH 2 CO 2 H (i) Assuming this reaction is the principal equilibrium, then the following relationship must hold true: [ H 2 NCH 2 CO 2 - ] = [ + H 3 NCH 2 CO 2 H ] ( ii ) Use this result and your answer to part c of Exercise 136 to calculate the pH at which equation (ii) is true. This pH will be the isoelectric point of glycine.

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Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

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Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 21, Problem 145CP
Textbook Problem
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The isoelectric point of an amino acid is the pH at which the molecule has no net charge. For glycine, that point would be the pH at which virtually all glycine molecules are in the form +H3NCH2CO2. This form of glycine is amphoteric since it can act as both an acid and a base. If we assume that the principal equilibrium at the isoelectric point has the best acid reacting with the best base present, then the reaction is

2 + H 3 NCH 2 CO 2 - H 2 NCH 2 CO 2 - + + H 2 NCH 2 CO 2 (i)

Assuming this reaction is the principal equilibrium, then the following relationship must hold true:

[ H 2 NCH 2 CO 2 - ]   =   [ + H 3 NCH 2 CO 2 H ] ( ii )

Use this result and your answer to part c of Exercise 136 to calculate the pH at which equation (ii) is true. This pH will be the isoelectric point of glycine.

Interpretation Introduction

Interpretation: The pH or isoelectric point of glycine at which the given equation is true is to be calculated.

Concept introduction: Amino acids are defined as organic compounds which contain amine (-NH2) and carboxyl (-COOH) functional groups, along with a side chain (R group) attached to each amino acid. The isoelectric point is defined as the pH of the molecule at which the molecule has no net electrical charge in statistical mean.

To determine: The pH or isoelectric point of glycine at which the given equation is true.

Explanation of Solution

Explanation

The equilibrium constant for the given reaction is 6.88×1013_ .

The equilibrium constant (Keq) of the reaction is given from the part (c) of Exercise 136. According to the part (c) of exercise 136, the equilibrium constant (Keq) of the reaction is Keq=6.88×1013 which is calculated as,

Given

The value of Ka(COOH) is 4.3×103 .

The value of Ka(NH3+) is 1.6×103 .

The equilibrium constant is calculated by using the expression,

Keq=Ka(COOH)×Ka(NH3+)

Substitute the value of Ka(COOH) and Ka(NH3+) in the above expression,

Keq=Ka(COOH)×Ka(NH3+)=4.3×103×1.6×1010=6.88×10-13_

Hence, Keq=6.88×1013_ .

By using equilibrium equation, the concentration of hydrogen ion at its isoelectric point is 8.3×107 M_ .

The equilibrium constant for the given reaction is 6.88×1013 .

The concentration of hydrogen ion at its isoelectric point is calculated by the equilibrium equation as follows,

Keq=[H+]2[H2NCH2CO2][+H2NCH2CO2H]

Substitute the value of Keq in the above expression, the concentration of hydrogen ion at its isoelectric point is calculated

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Chapter 21 Solutions

Chemistry: An Atoms First Approach
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