Question

Among the following oxoacids, the correct decreasing order of acid strength is

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> JEE MAINS 2014

• A. $HCl{O}_4>HOCl>HCl{O}_2>HCl{O}_3$
• B. $HCl{O}_4>HCl{O}_3>HCl{O}_2>HOCl$
• C. $HCl{O}_2>HCl{O}_4>HCl{O}_4>HCl{O}_3>HOCl$
• D. $HOCl>HCl{O}_2>HCl{O}_3>HCl{O}_4$

Answer 1

The correct option is B $HCl{O}_4>HCl{O}_3>HCl{O}_2>HOCl$

$HCl{O}_4\rightleftharpoons Cl{O}_4^{-}+H^{+}$
$HCl{O}_3\rightleftharpoons Cl{O}_3^{-}+H^{+}$
$HCl{O}_2\rightleftharpoons Cl{O}_2^{-}+H^{+}$
$HOCl\rightleftharpoons Cl{O}^{-}+H^{+}$

Resonance produces conjugate base.


$\text{(iv)}Cl{O}^{-}$ is not resonance stabilized.

As per resonance stability order of conjugate base is

$Cl{O}_4^{-}>Cl{O}_3^{-}>Cl{O}_2^{-}>Cl{O}^{-}$

Hence acidic strength order is

$HCl{O}_4>HCl{O}_3>HCl{O}_2>HClO$

Alternatively,
The oxidation number of chlorine atom in $HClO$ is +1
The oxidation number of chlorine atom in $HCl{O}_2$ is +3
The oxidation number of chlorine atom in $HCl{O}_3$ is +5
The oxidation number of chlorine atom in $HCl{O}_4$ is +7
With the increase in oxidation number of Cl atom from +1 to +7, the tendency of $Cl$ atom to attract the electron pair of$Cl-O$ bond towards itself increases then, the rupture of the $O-H$ bond becomes easier.
If the rupture of the $O-H$ bond increases, the amount of $H^{+}$ ions that furnishes will also increase.
So the correct order is
$HCl{O}_4>HCl{O}_3>HCl{O}_2>HClO$