Question

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

• A. $H_{2}S:p{K}_a=7.0$
• B. $HF:p{K}_{a}3.2$
• C. $H{N}_3:p{K}_a=4.7$
• D. $HCN:p{K}_a=9.2$

Answer 1

The correct option is D $HCN:p{K}_a=9.2$

$p{K}_a$ value is one method used 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 the acid dissociates more in water. A higher $p{K}_a$ value indicates a weaker acid.
$HCN$ has the highest $p{K}_a$ among all So, it is the weakest acid.