General Chemistry
General Chemistry
7th Edition
ISBN: 9780073402758
Author: Chang, Raymond/ Goldsby
Publisher: McGraw-Hill College
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Chapter 22, Problem 22.41QP
Interpretation Introduction

Interpretation:

The predominant form of glycine at pH 1,7 and 12 has to be predicted using the given equation.

Concept Introduction:

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

pH and pKa are related as,

    pH = pKa + log [conjugate base][acid]

Expert Solution & Answer
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Explanation of Solution

Given data:

    pKa of carboxyl group, COO = 2.3pKa of ammonium group, NH3+ = 9.6

At pH = 1,

Concentration of COOH

    pH = pKa + log [conjugate base][acid]1 = 2.3+ log [COO][COOH][COOH][COO] = 20 

Concentration of NH3+

    pH = pKa + log [conjugate base][acid]1 = 9.6+ log [NH2][NH3+][NH3+][NH2] = 4×108

Concentration of NH3+ is greater than COOH.  Hence, the predominant species is +NH3CH2COOH.

At pH = 7,

Concentration of COOH

    pH = pKa + log [conjugate base][acid]7 = 2.3+ log [COO][COOH][COO][COOH] = 5×104 

Concentration of NH3+

    pH = pKa + log [conjugate base][acid]7 = 9.6+ log [NH2][NH3+][NH3+][NH2] = 4×102

Concentration of COOH is greater than NH3+.  Hence, the predominant species is +NH3CH2COO.

At pH = 12,

Concentration of COOH

    pH = pKa + log [conjugate base][acid]12 = 2.3+ log [COO][COOH][COO][COOH] = 5×109 

Concentration of NH3+

    pH = pKa + log [conjugate base][acid]12 = 9.6+ log [NH2][NH3+][NH2][NH3+] = 2.5×102

Concentration of COOH is greater than NH3+.  Hence, the predominant species is NH2CH2COO.

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