Chapter 18, Problem 18.147P

(a)

Interpretation Introduction

Interpretation:

The disinfectant phenol (${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{OH}$) has ${\text{pK}}_{\text{a}}$ of $\text{10}\text{.0}$ in water but $\text{14}\text{.4}$ in methanol.  The reason has to be given for the different ${\text{pK}}_{\text{a}}$ values of phenol.

Concept Introduction:

The dissociation of an acid can be given as

  ${\text{HA + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{A}}^{\text{-}}{\text{ + H}}_{\text{3}}{\text{O}}^{\text{+}}$

The relationship between ${\text{pK}}_{\text{a}}$ and ${\text{K}}_{\text{a}}$ can be given as

  $\begin{array}{l}{\text{K}}_{\text{a}}{\text{ = [H}}^{\text{+}}{\text{][A}}^{\text{-}}\text{] / [HA]}\\ \\ {\text{pK}}_{\text{a}}{\text{= -log K}}_{\text{a}}\\ \\ {\text{pH = pK}}_{\text{a}}{\text{ = -logK}}_{\text{a}}\end{array}$

The ${\text{pK}}_{\text{a}}$ value increases with decrease in ${\text{K}}_{\text{a}}$ or the molarity of acid.

The ${\text{pK}}_{\text{a}}$ value decreases with increase in ${\text{K}}_{\text{a}}$ or the molarity of acid.

(b)

Interpretation Introduction

Interpretation:

The disinfectant phenol (${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\text{OH}$) has ${\text{pK}}_{\text{a}}$ of $\text{10}\text{.0}$ in water but $\text{14}\text{.4}$ in methanol.  Methanol is stronger or weaker base than water has to be given.

Concept Introduction:

The dissociation of an acid can be given as

  ${\text{HA + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{A}}^{\text{-}}{\text{ + H}}_{\text{3}}{\text{O}}^{\text{+}}$

The relationship between ${\text{pK}}_{\text{a}}$ and ${\text{K}}_{\text{a}}$ can be given as

  $\begin{array}{l}{\text{K}}_{\text{a}}{\text{ = [H}}^{\text{+}}{\text{][A}}^{\text{-}}\text{] / [HA]}\\ \\ {\text{pK}}_{\text{a}}{\text{= -log K}}_{\text{a}}\\ \\ {\text{pH = pK}}_{\text{a}}{\text{ = -logK}}_{\text{a}}\end{array}$

The ${\text{pK}}_{\text{a}}$ value increases with decrease in ${\text{K}}_{\text{a}}$ or the molarity of acid.

The ${\text{pK}}_{\text{a}}$ value decreases with increase in ${\text{K}}_{\text{a}}$ or the molarity of acid.

(c)

Interpretation Introduction

Interpretation:

The dissociation reaction of phenol in methanol has to be written.

Concept Introduction:

Dissociation reaction:

A chemical reaction in which a compound breaks into two or more parts is said to be a dissociation reaction.

  $\text{AB}\to \text{A+B}$

(d)

Interpretation Introduction

Interpretation:

The expression for the autoionization constant of methanol has to be written.

Concept Introduction:

Autoionization reaction:

The reaction that occurs between the molecules of same compound to produce ions without addition of any external reagent is said to be autoionization reaction.

Example:

Autoionization of water can be given as

  ${\text{H}}_{\text{2}}{\text{O + H}}_{\text{2}}\text{O }\rightleftarrows {\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{ + OH}}^{\text{-}}$