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UI CHEM 1120 - Weak Acid-Strong Base Titrations
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Chem 1120 1st Edition Lecture 17 Outline of Last Lecture I. Buffer CapacityII. Acid Base TitrationsIII.Titration CalculationsIV.Titration CurveOutline of Current Lecture I. Weak Acid-Strong Base Titrationa. 4 RegionsII. Diprotic Acids/Types of Acid and Base TitrationsIII.4 pH Graphs QuestionIV.IndicatorsCurrent LectureI. 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 NaOHNote, before calculations, divide the curve into the four regions as we did previously in the last lecture. a) Region 1: before titrant, weak acid problemThese 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 added0.1M X 25 mL = 2.5 mmol HAc0.1 M X 5 mL = 0.5 mmol NaOHHAc (aq) + OH- (aq) ——> H2O (l) + Ac- (aq)before 2.5mmol 0.5mmol 0mmolchange -0.5 -0.5 +0.5after 2mmol 0mmol 0.5mmolUse Henderson-Hasselbalch Equation:[base]/[acid] = (0.5mmol/30mL) / (2mmol/30mL) = 0.5/2 = 0.250pH = 4.745 + log(0.25) = 4.143Region 3: equivalence point, neutralization then weak base problem0.1M X 25 mL = 2.5 mmol NaOH0.1 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 MAc- + water ——> HAc + OH-(make another ICE table) end up with 0.05-X Ac-, X HAc, X OH-plug 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) = 5.278pH = 14 - pOH = 8.721Region 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 acidstrong acid added to strong basestrong base added to weak acidstrong base added to polyprotic acid IV. acid base indicator = a weak acid or weak base whose color differs in acidic and basic solutionsApply 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 = 9.1You want an indicator that will change color around the pH of the


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UI CHEM 1120 - Weak Acid-Strong Base Titrations

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