pH and pOH Concepts

  • pH Scale Overview:

    • Scale ranges from 0 to 14.
    • Acidic:
      - pH < 7 (e.g., soft drinks, lemon juice, coffee, laboratory acids)
    • Neutral:
      - pH = 7 (e.g., distilled water)
    • Basic:
      - pH > 7 (e.g., cleaning solutions, hand soap, baking soda)

  • pOH Concept:

    • pOH = -log[OH⁻]
    • INVERSE to pH scale; higher OH⁻ = lower pOH.
    • Examples:
    • [OH⁻] = 1 x 10⁻⁴ → pOH = 4
    • [OH⁻] = 1 x 10⁻² → pOH = 2

  • Key Relationship:

    • pH + pOH = 14

  • Equilibrium Constant (Kw):

    • Kw = [H⁺][OH⁻] = 1 x 10⁻¹⁴ at 25°C.

  • Temperature Effects on pH/pOH:

    • Kw changes with temperature, thus affecting pH/pOH:
    • At 40°C, Kw = 2.9 x 10⁻¹⁴ → pH ≈ 6.77 (neutral)
    • At 10°C, Kw = 2.9 x 10⁻¹⁵ → pH ≈ 7.27 (neutral)

  • Example Calculation:

    • Given pH = 9.32:
    1. [H⁺] = 10-pH = 4.79 x 10-10 moles/L
    2. pOH = 14 - pH = 4.68
    3. [OH⁻] = 10-pOH = 2.09 x 10-5 moles/L

  • Conclusion:

    • Understand acid-base relationships, pH/pOH calculations, and the impact of temperature on pH scale.
    • Practical implications with pH meters for experiments.