Equilibrium in a Solution of a Salt of a Weak Acid and a Weak Base

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) NH₄Cl

(d) Na₂HPO₄

(e) NH₄F

Solution

Consider each of the ions separately in terms of its effect on the pH of the solution, as shown here:

(a) The K⁺ cation and the Br⁻ anion are both spectators, since they are the cation of a strong base (KOH) and the anion of a strong acid (HBr), respectively. The solution is neutral.

(b) The Na⁺ cation is a spectator, and will not affect the pH of the solution; while the \({\text{HCO}}_{3}{}^{\text{−}}\) anion is amphiprotic. The K_a of \({\text{HCO}}_{3}{}^{\text{−}}\) is 4.7 \(×\) 10⁻¹¹,and its K_b is \(\cfrac{1.0\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-14}}{4.3\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-7}}\phantom{\rule{0.2em}{0ex}}=\phantom{\rule{0.2em}{0ex}}2.3\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-8}.\)

Since K_b >> K_a, the solution is basic.

(c) The \({\text{NH}}_{4}{}^{\text{+}}\) ion is acidic and the Cl⁻ ion is a spectator. The solution will be acidic.

(d) The Na⁺ cation is a spectator, and will not affect the pH of the solution, while the \({\text{HPO}}_{4}{}^{\text{2−}}\) anion is amphiprotic. The K_a of \({\text{HPO}}_{4}{}^{\text{2−}}\) is 4.2 \(×\) 10⁻¹³,

and its K_b is \(\cfrac{1.0\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-14}}{6.2\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-8}}\phantom{\rule{0.2em}{0ex}}=\phantom{\rule{0.2em}{0ex}}1.6\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-7}.\) Because K_b >> K_a, the solution is basic.

(e) The \({\text{NH}}_{4}{}^{\text{+}}\) ion is listed as being acidic, and the F⁻ ion is listed as a base, so we must directly compare the K_a and the K_b of the two ions. K_a of \({\text{NH}}_{4}{}^{\text{+}}\) is 5.6 \(×\) 10⁻¹⁰, which seems very small, yet the K_b of F⁻ is 1.4 \(×\) 10⁻¹¹, so the solution is acidic, since K_a > K_b.