Question 10.8: Determine the acid–base “parentage” of each of the following......
Determine the acid–base “parentage” of each of the following salts, and then use this information to predict whether each salt’s aqueous solution is acidic, basic, or neutral.
a. Sodium acetate, NaC_{2}H_{3}O_{2} b. Ammonium chloride, NH_{4}Cl
c. Potassium chloride, KCl d. Ammonium fluoride, NH_{4}F
a. The ions present are Na^{+} and C_{2}H_{3}O_{2} ^{-}. The “parent” base of Na^{+} is NaOH, a strong base. The “parent” acid of C_{2}H_{3}O_{2} ^{-} is HC_{2}H_{3}O_{2}, a weak acid. Thus the acid–base neutralization that produces this salt is
\underset{Strong base}{NaOH}+\underset{Weak acid}{HC_{2}H_{3}O_{2}}\longrightarrow H_{2}O+\underset{Weak acid–strong base salt}{NaC_{2}H_{3}O_{2}}
Strong base Weak acid Weak acid–strong base salt The solution of a weak acid–strong base salt (guideline 3) produces a basic solution.
b. The ions present are NH_{4}^{+} and Cl^{-}. The “parent” base of NH_{4}^{+} is NH_{3}, a weak base.
The “parent” acid of Cl^{-} is HCl, a strong acid. This “parentage” will produce a strong acid–weak base salt through neutralization. Such a salt gives an acidic solution upon hydrolysis (guideline 2).
c. The ions present are K^{+} and Cl^{-}. The “parent” base is KOH (a strong base), and the “parent” acid is HCl (a strong acid). The salt produced from neutralization involving this acid–base pair will be a strong acid–strong base salt. Such salts do not hydrolyze. The aqueous solution is neutral (guideline 1).
d. The ions present are NH_{4}^{+} and F^{-}. Both ions are of weak “parentage”; NH_{3} is a weak base, and HF is a weak acid. Thus NH_{4}F is a weak acid–weak base salt. This is a guideline 4 situation. In this situation, you cannot predict the effect of hydrolysis unless you know the relative strengths of the weak acid and weak base (which is the weaker of the two). HF has a K_{a} of 6.8 × 10^{-4} (Table 10.3). NH_{3} has a K_{b} of × 10^{-5} (Section 10.5).
Thus, NH_{3} is the weaker of the two and will hydrolyze to the greater extent, causing the solution to be acidic.
| TABLE 10.3 Ionization Constant Values (K_{a}) and Percent Ionization Values for 1.0 M Solutions, at 24 °C, of Selected Weak Acids | |||
| Name | Formula | K_{a} | Percent lonization |
| phosphoric acid | H_{3}PO_{4} | 7.5 × 10^{-3} | 8.3 |
| hydrofluoric acid | HF | 6.8 × 10^{-4} | 2.6 |
| nitrous acid | HNO_{2} | 4.5 × 10^{-4} | 2.1 |
| acetic acid | HC_{2}H_{3}O_{2} | 1.8 × 10^{-5} | 0.42 |
| carbonic acid | H_{2}CO_{3} | 4.3 × 10^{-7} | 0.065 |
| dihydrogen phosphate ion | H_{2}PO_{4}^{-} | 6.2 × 10^{-8} | 0.025 |
| hydrocyanic acid | HCN | 4.9 × 10^{-10} | 0.0022 |
| hydrogen carbonate ion | HCO_{3}^{-} | 5.6 × 10^{-11} | 0.00075 |
| hydrogen phosphate ion | HPO_{4}^{2-} | 4.2 × 10^{-13} | 0.000065 |