Question

Which of the following is the strongest acid? ($p{K}_a$ values given at ${25}^{o}C$)

• A. $HCOOH;p{K}_a=3.77$
• B. $C_6{H}_{5}COOH;p{K}_a=4.2$
• C. $C{H}_{3}COOH;p{K}_a=4.7$
• D. $C{H}_{3}C{H}_{2}COOH;p{K}_a=4.88$

Answer 1

The correct option is A $HCOOH;p{K}_a=3.77$

$p{K}_a$ value is to decide the strength of an acid. $p{K}_a$ is the negative log of the acid dissociation constant or $K_a$ value i.e. $p{K}_a=-\log K_a$.
A lower $p{K}_a$ value means higher $K_a$ which indicates a stronger acid. That is, it indicates that the acid dissociates more in water. A higher $p{K}_a$ value indicates a weaker acid.
$HCOOH$ has the lowest $p{K}_a$ among all. So, it is the strongest acid.