1. EXPERIMENT 25 PAGE 275 DR. SCOTT BUZBY PH.D. Determination of the Dissociation Constant of a Weak Acid

2. Objectives Become more familiar with the operation of a pH meter Quantitatively determine the equilibrium constant of a weak acid (K a )

3. 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

4. Acid-Base Equilibria (Weak Acids) By contrast, weak acids like acetic acid, HC 2 H 3 O 2 (abbreviated HOAc or HAc) will only partially dissociate The acid dissociation constant, K a (equilibrium constant) is used to determine the concentration of H + :

5. Acid-Base Titration Curve Equivalence Point ½ Equivalence Point

6. Equivalence Point At equivalence point the moles of base added equals the moles of acid in unknown sample:

7. 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 pK a of the unknown acid

8. Experimental Procedure Skip Part A – Page 279 Start with Part B – Page 280 Complete Parts C & D – Page 281

9. 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