Acids and Bases: Weak Acids, Weak Bases, and Salt Solutions

Weak Acids: Acid Dissociation Constant (Ka), % Ionization, and pH Calculations

Weak acids only partially ionize in aqueous solution. The extent of ionization is quantified by the acid dissociation constant, Ka. Calculating the pH of a weak acid solution involves setting up an equilibrium expression and often making simplifying assumptions.

• Acid Dissociation Constant (Ka): For a generic weak acid, HA:

• Example: Calculating the pH of a solution of acetylsalicylic acid (aspirin): - Find moles and molarity from mass and volume. - Set up an ICE table (Initial, Change, Equilibrium) for concentrations. - Assume x (amount ionized) is small compared to initial concentration for simplification. - If % ionization > 5%, use the quadratic formula to solve for x. - Calculate pH:

• % Ionization:

Using pH to Calculate Ka and % Ionization

If the pH of a weak acid solution is known, Ka can be calculated by determining [H3O+] from pH and using the equilibrium expression.

• Example: For formic acid, given pH and initial concentration, calculate Ka and % ionization.

Weak Bases: Base Dissociation Constant (Kb), % Ionization, pOH, and pH Calculations

Weak bases partially accept protons in water, producing hydroxide ions. The base dissociation constant, Kb, quantifies this equilibrium.

• Base Dissociation Constant (Kb): For a generic weak base, B:

• pKb: - Smaller pKb means a stronger base.

• % Ionization:

• pOH and pH:

• Example: Calculating the pH of a 0.30 M dimethylamine solution using Kb and the ICE table method.

Relationship Between Ka and Kb for Conjugate Acid-Base Pairs

For a conjugate acid-base pair in water, the product of their dissociation constants equals the ion-product constant for water, Kw.

• at 25°C

• This relationship allows calculation of Ka from Kb and vice versa.

• Example: If Kb for methylamine is known, Ka for methylammonium ion can be calculated as .

Conjugate Acids of Weak Bases and Conjugate Bases of Weak Acids

The conjugate acid of a weak base is itself a weak acid, and the conjugate base of a weak acid is a weak base. The acid-base properties of salts depend on the nature of their ions.

• Conjugate Acid Example: NH4+ is the conjugate acid of NH3 (a weak base) and is a weak acid.

• Conjugate Base Example: F– is the conjugate base of HF (a weak acid) and is a weak base.

• Cations from strong bases (e.g., Na+, K+) and anions from strong acids (e.g., Cl–, NO3–) are pH-neutral.

Calculating pH of Solutions Containing Conjugate Acids or Bases

To calculate the pH of a solution containing the conjugate acid of a weak base or the conjugate base of a weak acid:

• Calculate Ka (for conjugate acid) or Kb (for conjugate base) using .

• Set up an ICE table for the relevant equilibrium.

• Assume x is small if % ionization is less than 5%; otherwise, use the quadratic formula.

• Calculate [H3O+] or [OH–], then pH or pOH as appropriate.

Evaluating the Acid-Base Properties of Salt Solutions

The pH of a salt solution depends on the acid-base properties of its constituent ions. Salts can produce neutral, acidic, or basic solutions depending on the strengths of the parent acid and base.

• Case 1: Neutral Solutions - Salts from strong acids and strong bases (e.g., NaNO3) produce neutral solutions. - No hydrolysis occurs.

• Case 2: Basic Solutions - Salts from strong bases and weak acids (e.g., NaC2H3O2) produce basic solutions. - The anion hydrolyzes to produce OH–.

• Case 3: Acidic Solutions - Salts from weak bases and strong acids (e.g., NH4Cl) produce acidic solutions. - The cation hydrolyzes to produce H3O+. - Small, highly charged metal ions (e.g., Al3+) can also hydrolyze water to produce acidic solutions.

• Case 4: Salts with Both Weak Acid and Weak Base Components - The pH depends on the relative values of Ka and Kb. - If Ka > Kb, the solution is acidic. - If Kb > Ka, the solution is basic. - If Ka = Kb, the solution is neutral.

Summary Table: Common Weak Bases and Their Kb Values

The following table summarizes some common weak bases, their ionization reactions, and Kb values at 25°C:

Additional info: The above notes synthesize and expand upon the provided lecture slides, including definitions, calculation steps, and relevant examples for each concept. Images and tables are included only where they directly reinforce the explanation.