Chapter 16.3, Problem 4RC

Interpretation Introduction

Interpretation:

The ammonia ( $N{H}_3$) molecule $p{K}_{\text{a}}$ value should be calculate and identified.

Concept introduction:

pKa: It is introduced as an index to express the acidity of weak acids, where $p{K}_{\text{a}}$ is defined as fallows, we consider the $K_{\text{a}}$ constant for acetic acid $1.8\times {10}^{-5}(pKa)$ but $p{K}_{\text{a}}$ constant for is 4.8 which is a simpler expression. Furthermore the smaller pKa value has strongest acid.

In aqueous solution an acid undergoes ionization. The ionization of an acid is can be expressed in terms of equilibrium constant. The quantitative measurement tells about the strength of the acid. Higher the value of ${\text{K}}_a$ stronger will be the acid. The acid dissocition can be represented as following equilibrium,

$\text{HA}\left(\text{aq}\right){\text{+ H}}_{\text{2}}\text{O}\left(\text{l}\right)\rightleftharpoons {\text{ H}}_{\text{3}}{\text{O}}^{+}\left(\text{aq}\right){\text{+ A}}^{-1}\left(\text{aq}\right)$

The dissociation constant for the acid is ${\text{K}}_a$,

${\text{K}}_a=\frac{\left[{\text{H}}_{\text{3}}{\text{O}}^{+}\right]\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}$

For simplifications ${\text{pK}}_a$ value is used to find the acidic strength of acid. Which is calculated by taking negative logarithm of ${\text{K}}_a$.

${\text{pK}}_a=-\log \left({\text{K}}_a\right)$

The lower value of ${\text{pK}}_a$ of an acid, stronger will be the acid. Thus on comparing the ${\text{pK}}_a$ value it can be determined which acid is stronger one among them.