14.4 Hydrolysis of Salts

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

The ammonium ion is the conjugate acid of the weak base ammonia, NH₃, and so it will undergo acid ionization (or acid hydrolysis):

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

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 (NH₄⁺) and inert anions (Cl⁻), resulting in an acidic solution.

Calculating the pH of an Acidic Salt Solution

Aniline is an amine that is used to manufacture dyes. It is isolated as anilinium chloride, [C₆H₅NH₃]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?

C₆H₅NH₃⁺(aq) + H₂O(l) ⇌ H₃O⁺(aq) + C₆H₅NH₂(aq)

Solution:

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

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

Substituting these equilibrium concentration terms into the K_a expression gives

K_a = [C₆H₅NH₂][H₃O⁺]/[C₆H₅NH₃⁺]

2.3 × 10⁻⁵ = (x)(x)/(0.233-x)

Assuming x < 0.05 × 0.233M, i.e. x < 0.0116 M, the equation is simplified and solved for x:

2.3 × 10⁻⁵ = x²/0.233

x = 0.0023 M          ASSUMPTION VALID

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

pH = -log[H₃O⁺] = −log(0.0023 M) = 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 NH₄⁺ and NO₃⁻. Which is the stronger acid, C₆H₅NH₃⁺ or NH₄⁺?

 

Answer:

[H₃O⁺] = 7.5 × 10⁻⁶M; C₆H₅NH₃⁺ is the stronger acid.

Salts with Basic Ions

As another example, consider dissolving sodium acetate in water:

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, CH₃CO₂^–, is the conjugate base of acetic acid, CH₃CO₂H, and so its base ionization (or base hydrolysis) reaction is represented by

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 (CH₃CO₂^–), resulting in a basic solution.

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

Determine the acetic acid concentration in a solution with [CH₃CO₂^–] = 0.050 M and [OH⁻] = 2.5 × 10⁻⁶M at equilibrium.

The reaction is:

CH₃CO₂^–(aq) + H₂O(l) ⇌ CH₃CO₂H(aq) + OH⁻(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

Substituting the available values into the K_b expression gives

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

Check Your Learning:

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

 

Answer:

11.11