Chapter 5.3, Problem 11E

(a)

Interpretation Introduction

Interpretation: The 50 % ionization of benzoic acid at pH 4.2 needs to be determined if the pKa of benzoic acid is 4.2.

  

Concept Introduction: Bronsted and Lowry purposed the Bronsted-Lowry acid-base theory. It states that acid can give ${\text{H}}^{\text{+}}$ ions whereas a base can accept the ${\text{H}}^{\text{+}}$ ion in its solution. Hence this theory is entirely based on the presence of ${\text{H}}^{\text{+}}$ ion in the given substance.  It purposed the concept of conjugated acid-base pair. A Bronsted acid gives ${\text{H}}^{\text{+}}$ ion to form conjugated base whereas a Bronsted base accepts ${\text{H}}^{\text{+}}$ ion to form its conjugated acid.

  $\begin{array}{l}{\text{ HA + H}}_{\text{2}}\text{O }\underset{}{\to }{\text{ A}}^{\text{-}}{\text{ + H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \text{Bronsted + Bronsted Conjugated Conjugated}\\ \text{ Acid base base acid}\end{array}$

(b)

Interpretation Introduction

Interpretation: The reason of precipitation of benzoic acid needs to be explained if the pH of an aqueous solution of benzoate ion is lowered to pH 7 by adding acid.

  

Concept Introduction:Bronsted and Lowry purposed the Bronsted-Lowry acid-base theory. It states that acid can give ${\text{H}}^{\text{+}}$ ions whereas a base can accept the ${\text{H}}^{\text{+}}$ ion in its solution. Hence this theory is entirely based on the presence of ${\text{H}}^{\text{+}}$ ion in the given substance.  It purposed the concept of conjugated acid-base pair. A Bronsted acid gives ${\text{H}}^{\text{+}}$ ion to form conjugated base whereas a Bronsted base accepts ${\text{H}}^{\text{+}}$ ion to form its conjugated acid.

  $\begin{array}{l}{\text{ HA + H}}_{\text{2}}\text{O }\underset{}{\to }{\text{ A}}^{\text{-}}{\text{ + H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \text{Bronsted + Bronsted Conjugated Conjugated}\\ \text{ Acid base base acid}\end{array}$