Chapter 16, Problem 25PS

Interpretation Introduction

Interpretation:

A weak acid has a ${\text{K}}_{\text{a}}$ of $\text{6}{\text{.5×10}}^{\text{-5}}$. The ${\text{pK}}_{\text{a}}$ of the acid has to be determined.

Concept introduction:

Acid:

An acid is a substance that generates hydronium ions, ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ when added to the water.

Strong acid:

Strong acids are completely ionised in water.

For example,

$\text{HCl }\stackrel{{\text{H}}_{\text{2}}\text{O}}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{+ Cl}}^{\text{-}}$

Weak acid:

Weak acids are partially ionised in water.

For example,

${\text{HF + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{+ F}}^{\text{-}}$

${\text{pK}}_{\text{a}}$ is define as the negative logarithm of ${\text{K}}_{\text{a}}$