Question 2.5: Calculating Ka from pKa According to the data in Table 2.3, ...
Calculating K_{a} from pK_{a}
According to the data in Table 2.3, acetic acid has pK_{a} = 4.76. What is its K_{a}?
Strategy
Since pKa is the negative logarithm of K_{a}, it’s necessary to use a calculator with an ANTILOG or INV LOG function. Enter the value of the pK_{a} (4.76), change the sign (-4.76), and then find the antilog (1.74 \times 10^{-5}).
| TABLE 2.3 | ||||||
| Relative Strengths of Some Common Acids and Their Conjugate Bases | ||||||
| Conjugate | ||||||
| Acid | Name | pKa | base | Name | ||
| Weaker acid | CH_{3}CH_{2}OH | Ethanol | 16.00 | CH_{3}CH_{2}O^{-} | Ethoxide ion | Stronger base |
| \downarrow | H_{2}O | Water | 15.74 | HO^{-} | Hydroxide ion | \uparrow |
| HCN | Hydrocyanic acid | 9.31 | CN^{-} | Cyanide ion | ||
| H_{2}PO_{4}^{-} | Dihydrogen phosphate ion | 7.21 | HPO_{4}^{2-} | Hydrogen phosphate ion | ||
| CH_{3}CO_{2}H | Acetic acid | 4.76 | CH_{3}CO_{2}^{-} | Acetate ion | ||
| H_{3}PO_{4} | Phosphoric acid | 2.16 | H_{2}PO_{4}^{-} | Dihydrogen phosphate ion | ||
| HNO_{3} | Nitric acid | -1.3 | NO_{3}^{-} | Nitrate ion | ||
| HCI | Hydrochloric acid | -7.0 | Cl^{-} | Chloride ion | ||
| Strongeracid | Weaker base | |||||
K_{a}=1.74 \times 10^{-5}
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