Connect 1 Semester Access Card for General Chemistry: The Essential Concepts
Connect 1 Semester Access Card for General Chemistry: The Essential Concepts
7th Edition
ISBN: 9781259692543
Author: Raymond Chang Dr.; Kenneth Goldsby Professor
Publisher: McGraw-Hill Education
Question
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Chapter 22, Problem 22.47SP

(a)

Interpretation Introduction

Interpretation:

From the given the pKa values of the two groups, +NH3 and COOH of valine, the more acidic one has to be determined.

Concept Introduction:

Greater the pKa value of a substance lower will be its acidic property.

According to equation given in chapter 16 in the text book (equation 16.4),

pH and pKa are related as,

    pH=pKa+log[conjugatebase][acid]

(a)

Expert Solution
Check Mark

Answer to Problem 22.47SP

COOH group is more acidic.

Explanation of Solution

Given that pKa of +NH3 and COOH groups of valine are 9.62 and 2.32 respectively.

Since COOH has lower pKa value of 2.32, it is more acidic.

(b)

Interpretation Introduction

Interpretation:

The predominant form of valine at pH 1.0,7.0 and 12 has to be predicted.

Concept Introduction:

Greater the pKa value of a substance lower will be its acidic property.

According to equation given in chapter 16 in the text book (equation 16.4),

pH and pKa are related as,

    pH=pKa+log[conjugatebase][acid]

(b)

Expert Solution
Check Mark

Explanation of Solution

Given data:

    pKaofcarboxylgroup,COOH=2.3pKaofammoniumgroup,NH+3=9.6

At pH=1,

Concentration of COOH

    pH=pKa+log[conjugatebase][acid]1=2.32+log[COO][COOH][COOH][COO]=21 

Concentration of NH+3

    pH=pKa+log[conjugatebase][acid]1=9.62+log[NH2][NH+3][NH+3][NH2]=4.2×108

Concentration of NH+3 is greater than COOH.  Hence, the predominant species is CH(CH3)2CH(+NH3)COOH.

At pH=7,

Concentration of COOH

    pH=pKa+log[conjugatebase][acid]7=2.32+log[COO][COOH][COO][COOH]=4.8×104 

Concentration of NH+3

    pH=pKa+log[conjugatebase][acid]7=9.62+log[NH2][NH+3][NH+3][NH2]=4.2×102

Concentration of COO is greater than NH+3.  Hence, the predominant species is CH(CH3)2CH(+NH3)COO.

At pH=12,

Concentration of COOH

    pH=pKa+log[conjugatebase][acid]12=2.32+log[COO][COOH][COO][COOH]=4.8×109 

Concentration of NH+3

    pH=pKa+log[conjugatebase][acid]12=9.62+log[NH2][NH+3][NH2][NH+3]=2.4×102

Concentration of COO is greater than NH+3.  Hence, the predominant species is CH(CH3)2CH(NH2)COO.

(c)

Interpretation Introduction

Interpretation:

Isoelectric point of valine has to be calculated.

Concept Introduction:

Isoelectric point (pI) of an amino acid refers to the pH at which the amino acid remains neutral and does not migrate in an electric field

It is calculated as,

    pI=pKa1+pKa22

Where ,

    pKa1andpKa2 are two pKa values of amino acid.

(c)

Expert Solution
Check Mark

Answer to Problem 22.47SP

Isoelectric point of valine is calculated as 5.97

Explanation of Solution

Given data:

  pKa1ofcarboxylgroup,COOH,=2.32pKa2ofammoniumgroup,NH+3=9.62

Therefore isoelectronic point of valine is,

    pI=pKa1+pKa22=2.32+9.622=5.97

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