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. $B(p{K}_a=6.0)$
• C. $C(P{K}_a=4.0)$
• D. $D(K_a=1.0\times 10^{-5})$

Answer 1

Answer: (C)

$\because p{K}_a=-log{K}_a$
$\therefore p{K}_a$ for bezoic acid $=-log(6.3\times 10^{-5})$
$=-log6.3-log10^{-5}$
$=-0.799+5$
$=-0.799+5$
$=4.201$
$(a)p{K}_a$ for $A=-log(1.67\times 10^{-8})$
$=-log1.67-log10^{-8}$
$=-0.222+8=7.778$
$(b)p{K}_a$ for $B=6.0$
$(c)p{K}_a$ for $C=4.0$
$(d)p{K}_a$ for $D=-log1.0\times 10^{-5}$
$=-log1.0-log10^{-5}$
$=0+5=5$
For a stronger acids, $p{K}_a$ must be less.
Therefore, acid $C$ will be stronger than benzoic acid.