Some salts are composed of both acidic and basic ions, and so the pH of their solutions will depend on the relative strengths of these two species. Likewise, some salts contain a single ion that is amphiprotic, and so the relative strengths of this ion’s acid and base character will determine its effect on solution pH. For both types of salts, a comparison of the Ka and Kb values allows prediction of the solution’s acid-base status, as illustrated in the following example exercise.
Determining the Acidic or Basic Nature of Salts
Determine whether aqueous solutions of the following salts are acidic, basic, or neutral:
(a) KBr
(b) NaHCO3
(c) Na2HPO4
(d) NH4F
SolutionConsider each of the ions separately in terms of its effect on the pH of the solution, as shown here:
(a) The K+ cation is inert and will not affect pH. The bromide ion is the conjugate base of a strong acid, and so it is of negligible base strength (no appreciable base ionization). The solution is neutral.
(b) The Na+ cation is inert and will not affect the pH of the solution; while the $HCO_3^-$ anion is amphiprotic. The Ka of $HCO_3^-$ is $4.7×10^{-11}$, and its Kb is $\frac{1.0×10^{-14}}{4.3×10^{-7}}=2.3×10^{-8}$
Since Kb >> Ka, the solution is basic.
(c) The Na+ cation is inert and will not affect the pH of the solution, while the $HPO_4^{2-}$ anion is amphiprotic. The Ka of $HPO_4^{2-}$ is $4.2×10^{-13}$, and its Kb is $\frac{1.0×10^{-14}}{6.2×10^{-8}}=1.6×10^{-7}$
Because Kb >> Ka, the solution is basic.
(d) The $NH_4^+$ ion is acidic (see above discussion) and the F− ion is basic (conjugate base of the weak acid HF). Comparing the two ionization constants: Ka of $NH_4^+$ is $5.6×10^{-10}$ and the Kb of F− is $1.6×10^{-11}$, so the solution is acidic, since Ka > Kb.
Check Your LearningDetermine whether aqueous solutions of the following salts are acidic, basic, or neutral:
(a) K2CO3
(b) CaCl2
(c) KH2PO4
(d) (NH4)2CO3
Answer:
(a) basic; (b) neutral; (c) acidic; (d) basic