pH & pKa (College Board AP Chemistry)

pH

• pH is defined as -log[H₃O⁺]  
  • Note that [H⁺] is acceptable to use for AP chemistry
• To calculate [H₃O⁺] from pH we can use log^–pH

pK_a

• pK_a is a measure of the acidity or basicity of a substance in solution
  • pK_a = -log(K_a)
• To calculate K_a from pK_a we can use log^–pH

The relationship between pH and pK_a

• The pH of a solution can be related to the pK_a of an acid using the Henderson-Hasselbalch equation

pH = pK_a + log

• [A^–] is the concentration of the conjugate base
• [HA] is the concentration of the acid

pH curves, pH and pK_a

• During a titration buffer regions occur when 
  • Strong acid - weak base
  • Strong base - weak acid
• At this point, the buffer formed will resist changes in pH so the pH rises gradually as shown in the buffer region
• The half equivalence point is the stage of the titration at which exactly half the amount of weak acid has been neutralised
• At the half equivalence point, [acid] = [conjugate base] or [base] = [conjugate acid]
• For example during the titration of NaOH and ethanoic acid:
  
  • CH₃COOH + OH^–  CH₃COO^– + H₂O
  • CH₃COOH is the acid
  • CH₃COO^– is the conjugate base

The difference between pH and pK_a

• pH is a measure of the acidity or basicity of an aqueous solution
  • A pH below 7.00 indicates a substance is acidic 
  • A pH above 7.00 indicates a substance is basic
• pK_a is used to show the strength of an acid
  
  • A low pK_a value indicates a strong acid
  • A high pK_a value indicates a weak acid