Pharmaceutical Analytical Chemistry PC101 Lecture 6

Level 1 Pharmaceutical Analytical Chemistry (PC101) - Lecture 6

• Course offered by the Faculty of Pharmacy, Mansoura University
• Topics covered include the nature of acids and bases, acid-base theories, acid-base equilibrium, and pH calculations
• Important definitions discussed include:
  • Oxoacids: acids containing oxygen. Specifically, a compound containing hydrogen, oxygen and at least one other element, e.g., HNO₃, H₂SO₄, H₃PO₄.
  • Acid anhydrides (Acidic oxides): nonmetal oxides that react with water to form acids. These are considered air pollutants as they react with atmospheric water vapor
    • producing acid rain. Examples include CO₂, SO₂, SO₃.
  • Basic anhydrides: metal oxides that react with water to form bases. Examples include Na₂O, MgO, and CaO.

Acid-Base Equilibrium

• Law of Mass Action: The rate of a chemical reaction is proportional to the product of the active masses (molar concentrations) of the reacting substances. For example, aA + bB ⇌ cC + dD K = [C]^c[D]^d / [A]^a[B]^b where K is the equilibrium constant. Keq is affected only by temperature and pressure.

• In case of weak acids (e.g., CH₃COOH):

  • Kₐ = ionization constant of the acid or Acid dissociation constant
  • Kₐ = [CH₃COO⁻][H⁺]/[CH₃COOH]

• In case of weak bases (e.g., NH₄OH):

  • Kₕ = ionization constant of the base or base dissociation constant
  • Kₕ = [NH₄⁺][OH⁻]/[NH₄OH]

• In case of water (H₂O): Pure water is a very weak electrolyte, with very limited ionization.

  • H₂O ⇌ H⁺ + OH⁻
  • Dissociation constant of water (Kʷ) = [H⁺][OH⁻] / [H₂O] =1 x 10⁻¹⁴ (at 25°C)

• Relationship between pKₐ of an acid and pKₕ of its conjugate base:

  • pKₐ + pKₕ = pKʷ = 14

pH Calculations

• pH of a strong acid: pH = -log[H⁺]
• pH of a strong base: pOH = -log[OH⁻]; pH = 14 - pOH
• pH of a weak acid: pH = ½ (pKₐ + pCₐ) (Cₐ is the molar concentration of the acid)
• pH of a weak base: pOH = ½ (pKₕ + pCₕ ) and pH = 14 - pOH (Cₕ is the molar concentration of the base)
• pH of salts:
  • Salt of a strong acid and a strong base: pH = 7
  • Salt of a strong acid and a weak base: pH < 7
  • Salt of a weak acid and a strong base: pH > 7
  • Salt of a weak acid and a weak base: depends on pKₐ of the acid and pKₕ of the base
• pH problem solving diagram: Shows calculations for converting between [H⁺], [OH⁻], pH, and pOH.

Learning Outcomes

• The nature of acids and bases.
• Acid-Base Theories.
• Acid-Base Equilibrium.
• pH Calculations.

Homework Problems

• The document includes several homework problems relating to pH calculations. These problems involve various scenarios, including strong and weak acids and bases.