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Determination of the Dissociation Constant of a Weak Acid. Experiment 25 Page 275 Dr. Scott Buzby Ph.D. Objectives. Become more familiar with the operation of a pH meter Quantitatively determine the equilibrium constant of a weak acid (K a ). Acid-Base Equilibria (Strong Acids).

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determination of the dissociation constant of a weak acid

Determination of the Dissociation Constant of a Weak Acid

Experiment 25

Page 275

Dr. Scott Buzby Ph.D.

objectives
Objectives
  • Become more familiar with the operation of a pH meter
  • Quantitatively determine the equilibrium constant of a weak acid (Ka)
acid base equilibria strong acids
Acid-Base Equilibria (Strong Acids)
  • According to Brønsted-Lowry acid-base theory the strength of an acid is related to its ability to donate protons
  • A strong acid, HCl, will completely dissociate in dilute aqueous solutions
  • So the [H+] concentration of 0.1M HCl is 0.1M
acid base equilibria weak acids
Acid-Base Equilibria (Weak Acids)
  • By contrast, weak acids like acetic acid, HC2H3O2 (abbreviated HOAc or HAc) will only partially dissociate
  • The acid dissociation constant, Ka (equilibrium constant) is used to determine the concentration of H+:
acid base titration curve
Acid-Base Titration Curve

Equivalence Point

½ Equivalence Point

equivalence point
Equivalence Point
  • At equivalence point the moles of base added equals the moles of acid in unknown sample:
equivalence point1
½ Equivalence Point
  • ½ equivalence point is the point where ½ the volume of base added to reach equivalence point has been added
  • At the ½ equivalence point pH of the solution is equal to the pKa of the unknown acid
experimental procedure
Experimental Procedure
  • Skip Part A – Page 279
  • Start with Part B – Page 280
  • Complete Parts C & D – Page 281
due next week
Due Next Week
  • Report Sheet – Pages 283-285
  • Questions – Pages 285-286
  • 3 Graphs – Pages 287-289 (or Printouts)
  • Pre-Lab Experiment 27 – Page 313
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