Question

You may need Table 7.2 to answer the following questions.

a. Which is the stronger base, or H₂O?

b. Which is the stronger base, H₂O or NO₂^-?

c. Which is the stronger base, CN^-or OC₆H₅^-?

Short answer

The higher the K_a, i.e., the acid dissociation constant value, the stronger the acid is. Strong acids always have a weak conjugate base.

Basicity Order:

Cl^-< H₂O

H₂O <NO₂^-

CN^-<OC₆H₅^-

Step-by-step solution

Stronger Base out of Cl- and H2O

HCl is a much stronger acid than ${\mathbf{\text{H}}}_{\mathbf{3}}{\mathbf{O}}^{+}$ as it has high value of acid dissociation constant (K_a). As it is stronger than ${\mathbf{\text{H}}}_{\mathbf{3}}{\mathbf{O}}^{+}$, its conjugate base, i.e.${\mathbf{\text{Cl}}}^{-}$, is a weaker base than ${\mathbf{\text{H}}}_{\mathbf{2}}\mathbf{O}$.

Stronger Base out of H2O and NO2-

K_afor H₃O⁺is considered as 1.0, and that of${\text{HNO}}_2$ according to table 7.2 is Hence, ${\text{HNO}}_2$ is a weaker acid than${\text{H}}_3{O}^{+}$ . So, its conjugate base, i.e.,$\text{N}{{\text{O}}_2}^{-}$ is a stronger base than${\text{HNO}}_2$ .

Stronger Base out of CN- and OC6H5-

K_a values for$\text{HCN}$ and${\text{HOC}}_6{H}_5$ (phenol) according to table 7.2 are and respectively. Hence,$\text{HCN}$ is a stronger acid than phenol. Hence, the conjugate base of$\text{HCN}$ , i.e.,${\text{CN}}^{-}$ is weaker than the base as compared to the conjugate base of ${\text{HOC}}_6{H}_5 i.e.{,}^{-}O{C}_6{H}_5.$