pH & pKa (College Board AP® Chemistry)

pH & pKa

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