   Chapter 16, Problem 118IL

Chapter
Section
Textbook Problem

Amino acids are an important group of compounds. At low pH, both the carboxylic acid group (—CO2H) and the amine group (—NHR) are protonated. However, as the pH of the solution increases (say, by adding base), the carboxylic acid proton is removed, usually at a pH between 2 and 3. In a middle range of pHs, therefore, the amine group is protonated, but the carboxylic acid group has lost the proton. (This is called a zwitterion.) At more basic pH values, the amine proton is dissociated. What is the pH of a 0.20 M solution of alanine hydrochloride, [NH3CHCH3CO2H]Cl?

Interpretation Introduction

Interpretation:

The pH has to be calculated for 0.20M solution of Alanine Hydrochloride.

Concept introduction:

An amino acid has both basic amine group (NH2) and an acid carboxylic acid (COOH) group.

The Henderson-Hasselbalch equation is,

pH=pKa+log([CONJUGATEBASE][ACID])

pH is a measure of hydrogen ion concentration and is calculated as:

pH=log[H+]

Explanation

The pH of the solution of an Alanine is increases by adding a hydroxide ions, the hydrogen ion is removed from the NH3+ group.

CH3CH(NH3+)(COO)+OHCH3CH(NH2)(COO)+H2O

The pH of the solution of an alanine is decreases by adding an acid to a solution of alanine, the COO part of zwitter ion picks up a proton.

CH3CH(NH3+)(COO)+H+(aq)CH3CH(NH3+)(COOH)                  (3)

Given:

The concentration of Alanine Hydrochloride is ([COOH]+[COO])=0.20M    (4)

The pKa of carboxyl group of Alanine is 2.4.

The pKa of amine group of Alanine is 9.7.

The solution 0.20M Alanine HCl is composed of 0.20M Alanine and 0.20M HCl. Thus, there is an expectation to have an acidic pH and only the carboxyl group of Alanine is involved in proton equilibrium as in equation (3).

Substitute the value of acidic pKa in equation the below equation:

pH=pKa+log([CONJUGATEBASE][ACID])

pH=2

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