Question

You may need Table 7.2 to answer the following questions.

a. Which is the stronger acid, HCl or H₂O?

b. Which is the stronger acid, H₂O or HNO₂?

c. Which is the stronger acid, HCN or HOC₆H₅?

Short answer

The higher the K_a, i.e., the acid dissociation constant value, the stronger the acid is.

Acidity Order :

$$\begin{gathered} {\text{HCl>H}}_{2}O \\ {H}_{2}O<HN{O}_2 \\ HCN>HO{C}_6{H}_5 \end{gathered}$$

Step-by-step solution

The Value and Acidic Strength

Acid dissociation constant, i.e., K_atells us about the strength of an acid in solution. A higher value of K_a represents a stronger acid.

Stronger Acid out of HCland H2O  

HCl is a much stronger acid than water as it has a very high acid dissociation constant value (K_a).

Stronger Acid out of H2O and HNO2

K_a for water is 1.0´$\times$10^-14 and that of ${\text{HNO}}_2$ according to table 7.2 is$4.0\times {10}^{-4}$ . Hence,${\text{HNO}}_2$ is a stronger acid than ${\text{H}}_{2}O$.

Stronger Acid out of HCN and  HOC6H5

K_a values for HCN and HOC₆H₅ (phenol) according to table 7.2 are 6.2 ´$\times$ 10^-10 and 1.6´$\times$ 10^-10 respectively. Hence, HCN is a stronger acid than phenol.