Answer
a. $HCl$ is stronger.
b. $HNO_2$ is stronger.
c. $HCN$ is stronger.
Work Step by Step
Using the table 13-2, from page 531, and knowing that:
Strong acids: $K_a > 1$
Water: $K_a = 1.0 \times 10^{-14}$
a. $HCl$ is a strong acid: $K_a > 1$
$H_2O$: $K_a = 1.0 \times 10^{-14}$
$HCl$ has the greater $K_a$, so it is the stronger acid.
b. $HNO_2$ has $K_a = 4.0 \times 10^{-4}$
$H_2O$ has $K_a = 1.0 \times 10^{-14}$
$HNO_2$ has the greater $K_a$, so it is the stronger acid.
c. $HCN$ has $K_a = 6.2 \times 10^{-10}$
$HOC_6H_5$ has $Ka = 1.6 \times 10^{-10}$
$HCN$ has the greater $K_a$, so it is the stronger acid.
