How to Calculate pH Values

Key Takeaways

• pH Scale: Measures acidity or basicity of a solution.
  • Scale: 0 to 14
  • Neutral: 7
  • Acidic: < 7
  • Basic: > 7
• Formula: pH = -log₁₀[H⁺]
  • Negative logarithm of hydrogen ion concentration.

Understanding Acids and Bases

• pH is related to hydrogen ion concentration in aqueous solutions.
• Dissociation of Water:
  • H₂O → H⁺ + OH⁻
  • Equilibrium: Kw = [H⁺][OH⁻] = 1x10⁻¹⁴ at 25°C
• Solution Types:
  • Acidic: [H⁺] > 1x10⁻⁷
  • Basic: [H⁺] < 1x10⁻⁷

Calculating pH and [H⁺]

• Formulas:
  • pH = -log₁₀[H⁺]
  • [H⁺] = 10⁻ᵖᴴ
• Use scientific calculators with "log" function (not "ln") for calculations.

Relationship Between pH and pOH

• Formula: pH + pOH = 14
• Useful for calculating pH of bases by finding pOH first.

Example Problems

1. Calculate pH for [H⁺] = 1.4 x 10⁻⁵ M
   • Solution: pH = 4.85
2. Calculate [H⁺] from pH = 8.5
   • Solution: [H⁺] = 3.2 x 10⁻⁹ M
3. Find pH for [H⁺] = 0.0001 M
   • Solution: pH = 4
4. Find pH of 0.03 M HCl
   • Solution: pH = 1.5

Tips for pH Calculations

• Verify that calculated pH values make sense (e.g., acids should have low pH).
• pH values should typically be between 0 and 14 in practice.

References

• Important works by Covington, Bates, Durst, the International Union of Pure and Applied Chemistry, and Mendham et al.
• ThoughtCo Article by Anne Marie Helmenstine, Ph.D.