Pharmaceutical Analytical Chemistry PC101 Lecture 6

Study Notes

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.

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 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.

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