AP Chemistry Unit 8.7 pH and pKa

pH and pKa

  • Based on the pH of the environment, you can tell the relative ratio of conjugate base to acid
  • If pH < pKa, then the acid form has the higher concentration
    • Weak base & conj acid → conj acid has higher concentration
  • If pH > pKa, then the conjugate base form has the higher concentration
    • Weak base & conj acid → weak base has higher concentration

Acid-Base Indicators

  • Used in titrations to indicate when we have passed the equivalence point
  • Are weak acids that change color when they become bases
  • Endpoint: when the indicator changes color → tell us we have reached the eq point
  • Indicators have a “useful range” → have kA and pKa values
  • Useful indicator range =
    • Best indicator to choose for a particular titration will be the one whose useful range includes or is closest to the pH at equivalence for that particular titration
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