Question

The $pK _{ a }$ of a weak acid, $HA$, is $4.80$ and the $pK _{ b }$ of a weak base, $BOH$, is $4.78$. The $pH$ of an aqueous solution of the corresponding salt, $BA$, will be:

• A. 8.23
• B. 9.41
• C. 7.01
• D. 5.91

Answer 1

Answer: (C)

Let, the salt of weak acid and base like $CH _{3} COONH _{4}$ Now,
Salt hydrolysis formula is:
$ K _{ h } =\frac{ K _{ w }}{ K _{ a } K _{ b }}$
$h =\sqrt{\frac{ K _{ w }}{ K _{ a } K _{ b }}} $
$pH =\frac{1}{2}\left[ pK _{ w }+ pK _{ a }- pK _{ b }\right]$
So, $pH$ of Salt is,
$pH =\frac{1}{2}(14+4.80-4.78) $
$=7.01$
This is the required solution.