Question 7.1: Relative Base Strength Using Table 7.2, arrange the followin...
Relative Base Strength
Using Table 7.2, arrange the following species according to their strength as bases: H_{2}O , F^{-} , Cl^{-} , NO_2^{-}, and CN^{-} .
Table 7.2
Values of K_{a} for Some Common monoprotic Acids
| Formula | Name | Value of K_{a} | |
| HSO_{4}^{-} | Hydrogen sulfate ion | 1.2 × 10^{-2} |  |
| HClO_{2} | Chlorous acid | 1.2 × 10^{-2} | |
| HC_{2}H_{2}ClO_{2} | Monochloracetic acid | 1.35 × 10^{-3} | |
| HF | Hydrofluoric acid | 7.2 × 10^{-4} | |
| HNO_{2} | Nitrous acid | 4.0 × 10^{-4} | |
| HC _{2}H_{3} O_{2} | Acetic acid | 1.8 × 10^{-5} | |
| [Al( H_{2}O)_{6}] ^{3+} | Hydrated aluminum(III) ion | 1.4 × 10^{-5} | |
| HOCl | Hypochlorous acid | 3.5 × 10^{-8} | |
| HCN | Hydrocyanic acid | 6.2 × 10^{-10} | |
| NH_{4}^{+} | Ammonium ion | 5.6 × 10^{-10} | |
| HOC_{6}H_{5} | Phenol | 1.6× 10^{-10} |
Remember that water is a stronger base than the conjugate base of a strong acid but a weaker base than the conjugate base of a weak acid. This rule leads to the following order:
Cl^{-} < H_{2}O < conjugate bases of weak acids
Weakest bases → Strongest bases
We can order the remaining conjugate bases by recognizing that the strength of an acid is inversely related to the strength of its conjugate base. From Table 7.2 we have
K_{a} for HF > K_{a} for HNO_{2} > K_{a} for HCNThus the base strengths increase as follows:
F^{-} < NO_{2}^{-} < CN^{-}The combined order of increasing base strength is
Cl^{-} < H_{2}O < F^{-} < NO_{2}^{-} < CN^{-}