Q18.13P - Chemistry: Molecular Nature Of Matter And Change

Question

Question: Use Appendix C to rank the following in order of increasing acid strength: ${\mathbf{\text{HIO}}}_{\mathbf{\text{3}}}{\mathbf{\text{, HI, CH}}}_{\mathbf{\text{3}}}\mathbf{\text{COOH, HF}}$ .

Step-by-Step Solution

Answer

The order of compounds in increasing acidic strength is

$C{H}_{3}COOH\left(1.8\times {10}^{-5}\right)<HF\left(6.8\times {10}^{-4}\right)<HI{O}_3\left(1.6\times {10}^{-1}\right)<HI$

Step 1: Concept Introduction

The ability of an acid to lose its ${\mathbf{\text{H}}}^{\mathbf{\text{ + }}}$ ion is measured by its acid strength. A strong acid ionises completely in a solution, whereas a weak acid only ionises partially.

Step 2: Ranking of compounds in order of Increasing Acid Strength

First, list the $K_a$ values of the following compounds from appendix C –

${\text{HIO}}_{\text{3}}{\text{, HI, CH}}_{\text{3}}\text{COOH, HF}$

Compound	$K_a$
${\text{HIO}}_{\text{3}}$	$1.6\times {10}^{-1}$
HI	-
${\text{CH}}_{\text{3}}\text{COOH}$	$1.8\times {10}^{-5}$
HF	$6.8\times {10}^{-4}$

 

To rank the compounds in order of increasing acid strength, rank them in order of increasing $K_a$ . The $K_a$ of HI is not given in Appendix Cbecause it is known that it is the only strong acid and the rest are classified asweak acid. So, it will be ranked last (highest acid strength).

The order is as follows –

$1.8\times {10}^{-5}<6.8\times {10}^{-4}<1.6\times {10}^{-1}<HI$

$C{H}_{3}COOH<HF<HI{O}_3<HI$

Therefore, the ranking order is $C{H}_{3}COOH<HF<HI{O}_3<HI$.