Ch11-TCF

Ch11-TCF - Quantitative Chemical Analysis Chapter 11:...

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Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 11: Acid-Base Titrations Acid-Base Titrations
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1 What’s wrong with this explanation? Ribonuclease Three tyrosines have “normal” p K a ’s, ~10 Three tyrosines have p K a > 12 Explanation: ( Harris 7th Ed. , page 199) . . . “If tyrosine is buried deep inside the protein, it is not readily accessible and a high concentration of OH is required to remove the proton from the phenol group.” p K a ’s are thermodynamic parameters whereas this explanation is kinetic (rates of base migration to the interior of the enzyme).
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H + (g) + H 2 O (g) 695 kJ/mol H 3 O + (g) + aq 395 kJ/mol
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2 Consider titration of a strong base by a strong acid . A graph where two moles of HBr are gradually added to one mole of KOH in one liter of solution with no change in volume . (Thus, this is not a true titration.) Note the symmetry of the graph.
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• Now, we are titrating 50 mL of 0.0200 M KOH by gradual addition of 0.100 M HBr. • In a real titration, the volume changes continuously. Note that the concentrations of ALL species are changing (independent of reactions) as the volume of acid is added. Thus, the pH vs . V a (volume of acid added) plot is not “symmetrical.” •Point(b) on the graph corresponds to the equivalence point . What is the “ equivalence volume ,” V e ? – the volume where … mmoles of base = mmoles of acid (vol) b ×(conc) b = (vol) a ×(conc) a (50.00 mL) b ×(0.0200 M) b = V e × (0.100 M) a and V e = 10.00 mL 3
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4 In the (a) or Excess Base section of the graph Now, what is [OH ] after addition of 6.00 mL of acid? [OH
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Ch11-TCF - Quantitative Chemical Analysis Chapter 11:...

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