pH and pOH Concepts

pH and pOH

• Definition of pH:

  • pH is the measure of the acidity or basicity of a solution.
  • Mathematically expressed as:
  • pH = -log[H₃O⁺]

• Definition of pOH:

  • pOH is the measure of the concentration of hydroxide ions (OH⁻) in a solution.
  • Mathematically expressed as:
  • pOH = -log[OH⁻]

• Relationship Between pH and pOH:

  • The sum of pH and pOH in pure water at 25°C is always equal to 14.
  • Formula:
  • pH + pOH = 14.00

• Calculating pH:

  • Example:
  • Given nitrogenous concentration [H₃O⁺] = 1.0 × 10⁻⁷ M:
    • pH = -log(1.0 × 10⁻⁷) = 7.00
  • Example with different concentrations:
  • For [H₃O⁺] = 4.9 × 10⁻⁶ M:
    • pH = -log(4.9 × 10⁻⁶) ≈ 5.31

• Calculating pOH:

  • Example:
  • Given [OH⁻] = 1.0 × 10⁻⁷ M:
    • pOH = -log(1.0 × 10⁻⁷) = 7.00

• Dissociation of Water:

  • Water (H₂O) dissociates into hydrogen ions and hydroxide ions:
  • H₂O ⇌ H⁺ + OH⁻

• pH Scale:

  • Scale ranges from 0 to 14:
  • pH < 7: acidic
  • pH = 7: neutral
  • pH > 7: basic

• Application of pH in calculations:

  • Calculation of pH from hydroxide ion concentration:
  • Example:
    • Given [OH⁻] = 0.0125 M:
    • pOH = -log(0.0125) ≈ 1.903
    • pH = 14.00 - pOH = 14.00 - 1.903 ≈ 12.10

• Carbonic Acid Equilibrium:

  • CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid)
  • H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Acid-Base Neutralization Example:

  • H₂SO₄ + H₂O ⇌ H⁺ + HSO₄⁻
  • pH can also be analyzed through the equilibria of the acid-base reactions in the solution.

• Notes on Chemical Equilibrium in Solutions:

  • Different concentrations affect the pH levels as shown in various examples.
  • Understand how to inter-relate pH and pOH based on OH⁻ and H₃O⁺ concentrations for comprehensive examples.

• Real-World Applications:

  • The concepts of pH and pOH are crucial for biological systems, environmental science, and industrial applications where acidic and basic conditions significantly affect reactions and outcomes.