Chapter 15d: pH Calculations

Key Concepts

• pH Definition: pH is a scale used to specify the acidity or basicity of an aqueous solution.
• Importance of pH: Understanding pH is crucial for chemistry, biology, and environmental science.

pH Calculation Methods

• pH Formula:
  • pH = -log[H⁺]
  • Where [H⁺] is the concentration of hydrogen ions in moles per liter.
• pOH and pH Relation:
  • pH + pOH = 14 (at 25°C)
  • Useful in calculating pH when pOH is known.

Strong Acids and Bases

• Strong Acids:
  • Completely dissociate in water.
  • Example: HCl → H⁺ + Cl⁻
  • pH calculation: Direct use of initial concentration.
• Strong Bases:
  • Completely dissociate in water.
  • Example: NaOH → Na⁺ + OH⁻
  • pH calculation: Calculate pOH first and then convert to pH.

Weak Acids and Bases

• Weak Acids:
  • Partially dissociate in water.
  • Example: CH₃COOH ⇌ H⁺ + CH₃COO⁻
  • Requires equilibrium expressions for calculation.
• Weak Bases:
  • Partially dissociate in water.
  • Example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
  • Use Kb to find [OH⁻] and then calculate pOH.

Buffer Solutions

• Definition: Solutions that resist changes in pH upon the addition of small amounts of acid or base.
• Components:
  • Typically contain a weak acid and its conjugate base.
  • Example: Acetic acid and sodium acetate.
• Henderson-Hasselbalch Equation:
  • pH = pKa + log([A⁻]/[HA])
  • Useful for buffer pH calculations.

Special Cases

• Dilutions:
  • Affect the concentration and therefore the pH.
  • Use M₁V₁ = M₂V₂ to find new concentrations.
• Temperature Effects:
  • pH calculations assume standard conditions (25°C).
  • Changes in temperature can affect ionization and therefore pH.

Practice Problems

• Regular practice problems are recommended to master pH calculations.