Determination of the Dissociation Constant of a Weak Acid

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Determination of the Dissociation Constant of a Weak Acid

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Determination of the Dissociation Constant of a Weak Acid

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Determination of the Dissociation Constant of a Weak Acid

Experiment 25

Page 275

Dr. Scott Buzby Ph.D.

- Become more familiar with the operation of a pH meter
- Quantitatively determine the equilibrium constant of a weak acid (Ka)

- 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

- 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+:

Equivalence Point

½ Equivalence Point

- At equivalence point the moles of base added equals the moles of acid in unknown sample:

- 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

- Skip Part A – Page 279
- Start with Part B – Page 280
- Complete Parts C & D – Page 281

- Report Sheet – Pages 283-285
- Questions – Pages 285-286
- 3 Graphs – Pages 287-289 (or Printouts)
- Pre-Lab Experiment 27 – Page 313