6.3: Hydrolysis of Salt Solutions

Learning Objectives

By the end of this section, you will be able to:

• Predict whether a salt solution will be acidic, basic, or neutral
• Calculate the concentrations of the various species in a salt solution
• Describe the acid ionization of hydrated metal ions

Salts with Acidic Ions

Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt's constituent ions. For example, dissolving ammonium chloride in water results in its dissociation, as described by the equation

$${\text{NH}}_4\text{Cl}(s)\rightleftharpoons {\text{NH}}_4{}^{+}(aq)+{\text{Cl}}^{-}(aq)$$

The ammonium ion is the conjugate acid of the base ammonia, NH₃; its acid ionization (or acid hydrolysis) reaction is represented by

NH₄⁺(aq) + H₂O(l) ⇌ H₃O⁺(aq) + NH₃(aq)

Since ammonia is a weak base, K_b is measurable and K_a > 0 (ammonium ion is a weak acid).

The chloride ion is the conjugate base of hydrochloric acid, and so its base ionization (or base hydrolysis) reaction is represented by

$${\text{Cl}}^{-}(aq)+{\text{H}}_2\text{O}(l)\rightleftharpoons \text{HCl}(aq)+{\text{OH}}^{-}(aq)K_{\text{b}}=K_{\text{w}}\text{/}{K}_{\text{a}}$$

Since HCl is a strong acid, K_a is immeasurably large and K_b ≈ 0 (chloride ions don’t undergo appreciable hydrolysis).

Thus, dissolving ammonium chloride in water yields a solution of weak acid cations (${\text{NH}}_4{}^{+}$) and inert anions (Cl⁻), resulting in an acidic solution.

Example 14.15

Calculating the pH of an Acidic Salt Solution

Aniline is an amine that is used to manufacture dyes. It is isolated as anilinium chloride, $[{\text{C}}_6{\text{H}}_5{\text{NH}}_3]\text{Cl},$ a salt prepared by the reaction of the weak base aniline and hydrochloric acid. What is the pH of a 0.233 M solution of anilinium chloride

\[ \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_3{ }^{+}(a q)+\mathrm{H}_2 \mathrm{O}(l) \rightleftharpoons \mathrm{H}_3 \mathrm{O}^+(a q)+\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2(a q) \]

Solution

The K_a for anilinium ion is derived from the K_b for its conjugate base, aniline (see Appendix H):

\[ K_{\mathrm{a}}=\dfrac{K_{\mathrm{w}}}{K_{\mathrm{b}}}=\dfrac{1.0 \times 10^{-14}}{4.3 \times 10^{-10}}=2.3 \times 10^{-5} \]

Using the provided information, an Equilibrium table for this system is prepared:

	C5H5NH3^+	H2O	C6H5NH2	H3O^+
Equlibrium Concentration (M)	0.233		X	X

Substituting these equilibrium concentration terms into the K_a expression gives

\[ \mathrm{K}_a=\dfrac{\left[\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \right] \left[\mathrm{H}_3 \mathrm{O}^{+}\right]}{\left[\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_3^{+}\right]} \]

\[ \mathrm{2.3 \times 10^{-5}}=\dfrac{\mathrm{X}^2}{0.233} \]

X = 0.002

The ICE table defines x as the hydronium ion molarity, and so the pH is computed as

$$\text{pH}=-\text{log}[{\text{H}}_3{\text{O}}^{+}]=\text{−}\text{log}(0.0023)=2.64$$

Check Your Learning

What is the hydronium ion concentration in a 0.100-M solution of ammonium nitrate, NH₄NO₃, a salt composed of the ions ${\text{NH}}_4{}^{\text{+}}$ and ${\text{NO}}_3{}^{\text{−}}.$ Which is the stronger acid ${\text{C}}_6{\text{H}}_5{\text{NH}}_3{}^{\text{+}}$ or ${\text{NH}}_4{}^{\text{+}}?$

Answer:

[H₃O⁺] = 7.5 $\times$ 10⁻⁶ M; ${\text{C}}_6{\text{H}}_5{\text{NH}}_3{}^{\text{+}}$ is the stronger acid.

Salts with Basic Ions

As another example, consider dissolving sodium acetate in water:

$${\text{NaCH}}_3{\text{CO}}_2(s)\leftrightharpoons {\text{Na}}^{+}(aq)+{\text{CH}}_3{\text{CO}}_2{}^{-}(aq)$$

The sodium ion does not undergo appreciable acid or base ionization and has no effect on the solution pH. This may seem obvious from the ion's formula, which indicates no hydrogen or oxygen atoms, but some dissolved metal ions function as weak acids, as addressed later in this section.

The acetate ion, ${\text{CH}}_3{\text{CO}}_2{}^{-}\text{,}$ is the conjugate base of acetic acid, CH₃CO₂H, and so its base ionization (or base hydrolysis) reaction is represented by

$${\text{CH}}_3{\text{CO}}_2{}^{-}(aq)+{\text{H}}_2\text{O}(l)\rightleftharpoons {\text{CH}}_3{\text{CO}}_2\text{H}(aq)+\text{OH}-(aq)K_{\text{b}}=K_{\text{w}}\text{/}{K}_{\text{a}}$$

Because acetic acid is a weak acid, its K_a is measurable and K_b > 0 (acetate ion is a weak base).

Dissolving sodium acetate in water yields a solution of inert cations (Na⁺) and weak base anions ${\text{(CH}}_3{\text{CO}}_2{}^{-}),$ resulting in a basic solution.

Example 14.16

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

Determine the acetic acid concentration in a solution with $[{\text{CH}}_3{\text{CO}}_2{}^{\text{−}}]=0.050M$ and [OH⁻] = 2.5 $\times$ 10⁻⁶ M at equilibrium. The reaction is:

NaCH₃CO₂(s) ⇋ Na⁺(aq) + CH₃CO₂⁻(aq)

Solution The provided equilibrium concentrations and a value for the equilibrium constant will permit calculation of the missing equilibrium concentration. The process in question is the base ionization of acetate ion, for which

\[ K_{\mathrm{b}}\left( \mathrm{CH}_3 \mathrm{CO}_2^{-}\right) = \dfrac{K_{\mathrm{w}}}{K_{\mathrm{a}}}=\frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}}=5.6 \times 10^{-10} \]

Substituting the available values into the K_b expression gives

\[\begin{gathered}K_{\mathrm{b}}=\frac{\left[\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\right]\left[\mathrm{OH}^{-}\right]}{\left[\mathrm{CH}_3 \mathrm{CO}_2^{-}\right]}=5.6 \times 10^{-10} \\ =\frac{\left[\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\right]\left(2.5 \times 10^{-6}\right)}{(0.050)}=5.6 \times 10^{-10}\end{gathered} \]

Solving the above equation for the acetic acid molarity yields [CH₃CO₂H] = 1.1 $\times$ 10⁻⁵ M.

Check Your Learning

What is the pH of a 0.083-M solution of NaCN?

Answer:

11.11

Salts with Acidic and Basic Ions

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 K_a and K_b values allows prediction of the solution’s acid-base status, as illustrated in the following example exercise.

Example 14.17

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

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 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 ${\text{HCO}}_3{}^{\text{−}}$ anion is amphiprotic. The K_a of ${\text{HCO}}_3{}^{\text{−}}$ is 4.7 $\times$ 10⁻¹¹,and its K_b is $\frac{1.0\times {10}^{\mathrm{-14}}}{4.3\times {10}^{\mathrm{-7}}}=2.3\times {10}^{\mathrm{-8}}.$

Since K_b >> K_a, the solution is basic.

(c) The Na⁺ cation is inert and will not affect the pH of the solution, while the ${\text{HPO}}_4{}^{\text{2}}{\text{−}}^{}$ anion is amphiprotic. The K_a of ${\text{HPO}}_4{}^{\text{2}}{\text{−}}^{}$ is 4.2 $\times$ 10⁻¹³,

and its K_b is $\frac{1.0\times {10}^{\mathrm{-14}}}{6.2\times {10}^{\mathrm{-8}}}=1.6\times {10}^{\mathrm{-7}}.$ Because K_b >> K_a, the solution is basic.

(d) The ${\text{NH}}_4{}^{\text{+}}$ ion is acidic (see above discussion) and the F⁻ ion is basic (conjugate base of the weak acid HF). Comparing the two ionization constants: K_a of ${\text{NH}}_4{}^{\text{+}}$ is 5.6 $\times$ 10⁻¹⁰ and the K_b of F⁻ is 1.6 $\times$ 10⁻¹¹, so the solution is acidic, since K_a > K_b.

Check Your Learning

Determine 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