Question

The aqueous solution of following salt will have the lowest $pH$ :

• A. $NaClO_3$
• B. $NaClO$
• C. $NaClO_2$
• D. $NaClO_4$

Answer 1

Answer: (D)

The salts of strong acids give acidic solution (having pH less than 7).

In the given salts, acid strength of oxoacids of chlorine increases with increase in oxidation number of chlorine.

Oxidation number $( x )$ of $Cl$ in $HClO _{4}$ (perchloric acid)

$1+x-8 =0 $

$\Rightarrow \, x =+7$

Oxidation number of $Cl$ in $HClO _{3}$ (chloric acid)

$1+x-6=0$

$\Rightarrow x=+5$

Oxidation number of Cl in $HClO _{2}$ (chlorous acid)

$ 1+x-4 =0 $

$\Rightarrow \, x =+3$

Oxidation number of $Cl$ in $HClO$ (hypochlorous acid)

$=1+x-2=0 $

$\Rightarrow \,x=+1$

So, acidic strength decreases in the order :

$\underset{\text{Most acidic}}{{HClO_4}}>\, HClO_3 >\,HClO_2 >\, \underset{\text{Less acidic}}{{HClO}}$

So, $HClO _{4}$ is the strongest acid and salt of this acid $NaClO _{4}$ will have the lowest $pH$.