Chapter 15.5, Problem 2RCF

Interpretation Introduction

Interpretation:

Using the given $\text{pH}$ of a weak acid, the value of ${\text{K}}_{\text{a}}$ has to be found.

Concept Introduction:

The acid which can dissociate only one proton is known as weak acid. It can be represented as $\text{HA}$.

The $\text{pH}$ of the monoprotic acid can be determined as follows:

Step 1: Using the ICE table, determine the equilibrium concentrations of all the chemical species involved in the dissociation of the monoprotic acid.

Step 2: In the ICE table, the change in concentration is the concentration of the ${\text{H}}^{\text{+}}$ ion that has to be determined from the $\text{pH}$ of the weak monoprotic acid as follows:

$\begin{array}{l}\text{pH}\text{=}-{\log }_{10}\left[{\text{H}}^{\text{+}}\right]\\ \left[{\text{H}}^{\text{+}}\right]={10}^{\text{-pH}}\end{array}$

Step 3: Substitute the equilibrium concentrations in the expression for the acid dissociation constant ${\text{K}}_{\text{a}}$.

Step 4: The value of ${\text{K}}_{\text{a}}$ can be obtained by evaluating the expression of ${\text{K}}_{\text{a}}$.