Chapter 9, Problem 55P

Interpretation Introduction

Interpretation:

The ${\text{K}}_{\text{a}}$ value for weak acid HA in water for the given equation and molecular art needs to be determined.

Concept Introduction:

The Bronsted-Lowry acid-base theory was purposed by Bronsted and Lowery is called 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.

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

A strong acid shows complete dissociation to respective anion and ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ whereas a weak acid can only partially ionize to its respective ions. A strong acid forms a weak conjugated base whereas a weak acid forms a strong conjugated base.

The strength of acid can be determined with the help of acid dissociation constant also. For acid HA the acid dissociation constant in its aqueous solution can be written as:

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