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

Weak Acids: 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 Calculation: Dissolving three aspirin tablets (acetylsalicylic acid, HC9H7O4) in 356 mL of water yields a 0.0152 M solution. Given , set up the equilibrium and solve for [H3O+]. If the percent ionization is greater than 5%, use the quadratic formula for accuracy.

• Percent Ionization:

• pH Calculation:

• Reverse Calculation: If pH is known, can be calculated using the equilibrium concentrations derived from the pH.

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

Weak bases partially accept protons in water, and their strength is measured by the base dissociation constant, Kb. Calculations for weak bases mirror those for weak acids, with the focus on hydroxide ion concentration.

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

• pOH and pH:

• Percent Ionization:

• pKb:

• Example Calculation: For 0.30 M dimethylamine (), set up the equilibrium, solve for [OH-], and calculate pOH and pH. If % ionization is less than 5%, the simplifying assumption is valid.

Relationship Between Ka and Kb for Conjugate Acid-Base Pairs

For a conjugate acid-base pair, the product of their dissociation constants equals the ion-product constant for water ():

• at 25°C

• This relationship allows calculation of if is known, and vice versa.

• Example: If for methylamine is , then for its conjugate acid is

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 these species are important in predicting the pH of salt solutions.

• Conjugate Acid Example: NH4+ (from NH3) is a weak acid.

• Conjugate Base Example: F- (from HF) is a weak base.

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

Evaluating the Acid-Base Properties of Salt Solutions

The pH of a salt solution depends on the acid-base properties of its constituent ions. Four main cases are considered:

• Case 1: Neutral Solutions – Salts from strong acids and strong bases (e.g., NaNO3) yield neutral solutions.

• Case 2: Basic Solutions – Salts from strong bases and weak acids (e.g., NaC2H3O2) yield basic solutions due to anion hydrolysis.

• Case 3: Acidic Solutions – Salts from weak bases and strong acids (e.g., NH4Cl) or small, highly charged metal ions (e.g., Al(NO3)3) yield acidic solutions due to cation hydrolysis.

• Case 4: Both Ions React – Salts from weak acids and weak bases (e.g., NH4F) can yield acidic, basic, or neutral solutions depending on the relative values of and :

  • If , solution is acidic.

  • If , solution is basic.

  • If , solution is neutral.

Table: Some Common Weak Bases

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

Summary of Key Equations

• at 25°C

• (for acids)

• (for bases)

Additional info: The ICE (Initial, Change, Equilibrium) table method is essential for setting up equilibrium calculations for weak acids and bases. When the simplifying assumption (that x is negligible) is not valid, the quadratic formula must be used to solve for equilibrium concentrations.