Question

Which of the following acids is stronger than benzoic acid $ (K_a = 6.3 \times 10^{-5}) $ ?

• A. $ A K_a = 1.67 \times 10^{-8} $
• B. $ B(pK_a = 6.0) $
• C. $ C (PK_a = 4.0) $
• D. $ D(K_a = 1.0 \times 10^{-5}) $

Answer 1

Answer: (C)

$\because pK_a = - log\,K_a$
$\therefore pK_a$ for bezoic acid $ = -log (6.3 \times 10^{-5})$
$ = -log \,6.3 - log \,10^{-5}$
$ = -0.799 + 5$
$ = -0.799 + 5$
$ = 4.201$
$(a)\, pK_a$ for $A = - log (1.67 \times 10^{-8})$
$= - log \,1.67- log \,10^{-8}$
$= - 0.222 + 8 = 7.778$
$(b) \,pK_a$ for $B = 6.0$
$(c)\, pK_a$ for $C = 4.0$
$(d)\, pK_a$ for $D = - log \,1.0 \times 10^{-5}$
$= -log \,1 .0 - log \,10^{-5}$
$= 0 + 5 = 5$
For a stronger acids, $pK_a$ must be less.
Therefore, acid $C$ will be stronger than benzoic acid.