Question 17.7: Obtaining Ka from Kb or Kb from Ka Use Tables 17.1 and 17.2 ...
Obtaining K_{a} from K_{b} or K_{b} from K_{a}
Use Tables 17.1 and 17.2 to obtain the following at 25^{\circ} \mathrm{C} : a. K_{b} for \mathrm{CN}^{-}; b. K_{a} for \mathrm{NH}_{4}^{+}.
Table 17.1
Acid-Ionization Constants at 25°C*
Substance | Formula | K_a |
Acetic acid | \mathrm{HC}_2 \mathrm{H}_3 \mathrm{O}_2 | 1.7 \times 10^{-5} |
Benzoic acid | \mathrm{HC}_7 \mathrm{H}_5 \mathrm{O}_2 | 6.3 \times 10^{-5} |
Boric acid | \mathrm{H}_3 \mathrm{BO}_3 | 5.9 \times 10^{-10} |
Carbonic acid | \mathrm{H}_2 \mathrm{CO}_3 | 4.3 \times 10^{-7} |
\mathrm{HCO}_3{ }^{-} | 4.8 \times 10^{-11} | |
Cyanic acid | \mathrm{HOCN} | 3.5 \times 10^{-4} |
Formic acid | \mathrm{HCHO}_2 | 1.7 \times 10^{-4} |
Hydrocyanic acìd | \mathrm{HCN} | 4.9 \times 10^{-10} |
Hydrofluoric acid | \text { HF } | 6.8 \times 10^{-4} |
Hydrogen sulfate ion | \mathrm{HSO}_4{ }^{-} | 1.1 \times 10^{-2} |
Hydrogen sulfide | \mathrm{H}_2 \mathrm{~S} | 8.9 \times 10^{-8} |
\mathrm{HS}^{-} | 1.2 \times 10^{-13†} | |
Hypochlorous acid | \mathrm{HClO} | 3.5 \times 10^{-8} |
Nitrous acid | \mathrm{HNO}_2 | 4.5 \times 10^{-4} |
Oxalic acid | \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4 | 5.6 \times 10^{-2} |
\mathrm{HC}_2 \mathrm{O}_4{ }^{-} | 5.1 \times 10^{-5} | |
Phosphoric acid | \mathrm{H}_3 \mathrm{PO}_4 | 6.9 \times 10^{-3} |
\mathrm{H}_2 \mathrm{PO}_4{ }^{-} | 6.2 \times 10^{-8} | |
\mathrm{HPO}_4{ }^{2-} | 4.8 \times 10^{-13} | |
Phosphorous acid | \mathrm{H}_2 \mathrm{PHO}_3 | 1.6 \times 10^{-2} |
\mathrm{HPHO}_3{ }^{-} | 7 \times 10^{-7} | |
Propionic acid | \mathrm{HC}_3 \mathrm{H}_5 \mathrm{O}_2 | 1.3 \times 10^{-5} |
Pyruvic acid | \mathrm{HC}_3 \mathrm{H}_3 \mathrm{O}_3 | 1.4 \times 10^{-4} |
Sulfurous acid | \mathrm{H}_2 \mathrm{SO}_3 | 1.3 \times 10^{-2} |
\mathrm{HSO}_3{ }^{-} | 6.3 \times 10^{-8} | |
*The ionization constants for polyprotic acids are for successive ionizations. For example, for \mathrm{H}_3 \mathrm{PO}_4, the equilibrium is \mathrm{H}_3 \mathrm{PO}_4+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{H}_2 \mathrm{PO}_4{ }^{-}. For \mathrm{H}_2 \mathrm{PO}_4{ }^{-}, the equilibrium is \mathrm{H}_2 \mathrm{PO}_4{ }^{-}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{HPO}_4{ }^{2-}. \dagger This value is in doubt. Some evidence suggests that it is about 10^{-19}. See R.J. Myers, J. Chem. Educ., 63, 687 (1986). |
Table 17.2
Base-Ionization Constants at 25°C
Substance | Formula | K_b |
Ammonia | \mathrm{NH}_3 | 1.8 \times 10^{-5} |
Aniline | \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 | 4.2 \times 10^{-10} |
Dimethylamine | \left(\mathrm{CH}_3\right)_2 \mathrm{NH} | 5.1 \times 10^{-4} |
Ethylamine | \mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2 | 4.7 \times 10^{-4} |
Hydrazine | \mathrm{~N}_2 \mathrm{H}_4 | 1.7 \times 10^{-6} |
Hydroxylamine | \mathrm{NH}_2 \mathrm{OH} | 1.1 \times 10^{-8} |
Methylamine | \mathrm{CH}_3 \mathrm{NH}_2 | 4.4 \times 10^{-4} |
Pyridine | \mathrm{C}_5 \mathrm{H}_5 \mathrm{~N} | 1.4 \times 10^{-9} |
Urea | \mathrm{NH}_2 \mathrm{CONH}_2 | 1.5 \times 10^{-14} |
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