Chem 1120 1st Edition Lecture 17 Outline of Last Lecture I Buffer Capacity II Acid Base Titrations III Titration Calculations IV Titration Curve Outline of Current Lecture I Weak Acid Strong Base Titration a 4 Regions II Diprotic Acids Types of Acid and Base Titrations III 4 pH Graphs Question IV Indicators Current Lecture I Weak Acid Strong Base Titration Example Analyte 25 mL of 0 1 M HAc HAc CH3COOH acetic acid Ka 1 8 X 10 5 pKa 4 745 Titrant x mL of 0 1 M NaOH Note before calculations divide the curve into the four regions as we did previously in the last lecture a Region 1 before titrant weak acid problem These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Point 2 before equivalence point buffer problem 5 mL 0 1 M NaOh is added 0 1M X 25 mL 2 5 mmol HAc 0 1 M X 5 mL 0 5 mmol NaOH HAc aq OH aq H2O l Ac aq before change after 2 5mmol 0 5 2mmol 0 5mmol 0mmol 0 5 0 5 0mmol 0 5mmol Use Henderson Hasselbalch Equation base acid 0 5mmol 30mL 2mmol 30mL 0 5 2 0 250 pH 4 745 log 0 25 4 143 Region 3 equivalence point neutralization then weak base problem 0 1M X 25 mL 2 5 mmol NaOH 0 1 X 25 mL 2 5 mmol HAc make ICE table end up with 0 mmol HAc 0 mmol OH 2 5 mmol Ac Ac 2 5mmol 25mL 25mL 0 05 M Ac water HAc OH make another ICE table end up with 0 05 X Ac X HAc X OHplug into the Kb equation Kb Kw Ka you end up getting x OH 5 27 X 10 6 which means the pOH is log 5 27 X 10 6 5 278 pH 14 pOH 8 721 Region 4 after equivalence point limiting reagent strong base problem 25 1 mL 0 1 M NaOH is added II With weaker acids the initial pH is higher and pH changes near the equivalence point are more subtle Titration of a diprotic acid with a strong base gives an equivalence point for each dissociation therefore leaving two vertical regions in the graph rather than one There are many types of titrations Acid weak or strong Acid monoprotic diprotic Base weak or strong Base monobasic dibasic Analyte acid or base III strong base added to strong acid strong acid added to strong base strong base added to weak acid strong base added to polyprotic acid IV acid base indicator a weak acid or weak base whose color differs in acidic and basic solutions Apply Le Chatelier s Principle Choosing an indicator Applying the Henderson Hasselbalch Equation Example Titrant 0 1 M NaOH strong base Analyte 0 1 M HAc weak acid At equivalence point pH 8 721 which indicator would be best Congo Red with pKa 4 or Phenolphthalein with pKa 9 1 You want an indicator that will change color around the pH of the equivalence point
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