Lecture Notes: Calculating pH of Weak Acids and Bases

Key Concepts

• pH and pOH Calculations:

  • For strong acids like HCl, which dissociates completely, use:
    • pH: -log[H₃O⁺]
  • For strong bases, use:
    • pOH: -log[OH⁻]
  • At 25°C:
    • pH + pOH = 14

• Weak Acids and Bases:

  • Weak acids do not dissociate completely, use a double arrow for equilibrium.
  • Calculate using Ka (acid dissociation constant):
    • Ka is expressed as [Products]/[Reactants]
    • Water is a liquid and not included in the expression.

• ICE Tables:

  • Used to calculate equilibrium concentrations in weak acid/base reactions.
  • Stands for Initial, Change, Equilibrium.

• Relation Between Ka and Kb:

  • Ka × Kb = Kw (1 x 10⁻¹⁴) at 25°C.

Calculating pH for Weak Acid

• Example: Acetic Acid (0.75 M)
  • Dissociation in water produces H₃O⁺ and acetate.
  • Use ICE table to find changes in concentrations.
  • Set up equilibrium expression using Ka.
  • For small Ka, ignore x in denominator for simplicity.
  • Solve quadratic for x, representing [H₃O⁺].
  • Calculate pH: -log[H₃O⁺].

Calculating pH for Weak Base

• Example: Ammonia (0.25 M)
  • Generates OH⁻ in water.
  • Use Kb to determine equilibrium concentrations.
  • Calculate pOH and then pH as 14 - pOH.

Salts and pH

• Example: Ammonium Chloride

  • NH₄⁺ acts as a weak acid.
  • Use Ka calculated from Kb for reactions involving NH₄⁺.
  • Use ICE table to find equilibrium concentrations.
  • Calculate pH from [H₃O⁺] using Ka.

• Example: Sodium Fluoride

  • F⁻ acts as a weak base.
  • Set up reaction with water, producing HF and OH⁻.
  • Use Kb calculated from Ka of HF.
  • Calculate pH from [OH⁻] using Kb.

Percent Ionization

• Formula: (x / [HA]) × 100%

  • x is concentration of ionized acid/base.

• Example Problem: Hydrofluoric Acid (HF)

  • Use ICE table and Ka to find [H₃O⁺].
  • Calculate percent ionization from x, initial concentration.

Important Formulas

• pH from [H₃O⁺]: pH = -log[H₃O⁺]
• pOH from [OH⁻]: pOH = -log[OH⁻]
• pH + pOH = 14 at 25°C
• Ka × Kb = Kw
• Percent Ionization: (x / [Initial Acid/Base]) × 100%