Chapter 19-Buffers - Ionic Equilibria Buffers and...

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Ionic Equilibria: Buffers and Titrations I. Buffers: Equilibria In Action A. Buffers Challenge : Maintaining pH within a known range is essential in many systems such as the blood, cells and manufacturing processes. How might pH changeover time in thesesystems? What would be the consequence? B. Composition of a buffer 1. Acidic buffers: 2. Basic buffers: Practice : Which of the following would be a buffer system? H 2 CO 3 plus added NaHCO 3 NaHSO 4 plus added, Na 2 SO 4 HSO 4 - plus added HSO 3 - Practice : For each substancelisted below, give the other substancenecessary to form a buffer. H 3 PO 4 and ______________ ______________ and NaHSO 3 NH 4 OH and ______________ ______________ and Na 2 HPO 4 CU C1131 – Baker RAP Buffers and Titrations Dr. Christine Kelly Modified 1.25.07 Page 1 of 19
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Practice : When the following solutions are mixed, would a buffer result? Mixture 1: Equal volumesof 0.1M HCl and 0.1M NaOH Mixture 2: 100. mL 0.1M acetic acid and 10.0 mL of 0.1M NaOH Mixture 3: Equal volumesof 0.1M NH 4 OH and 0.1M NH 4 NO 3 Mixture 4: 0.20 mol HPO 4 2- added to 0.25 mol H 2 PO 4 - in water. Mixture 5: 200. mL of 0.1M NaOH and 300.0 mL 0.1M acetic acid. Mixture 6: 200. mL of 0.1M NaOH and 100.0 mL 0.1M acetic acid. Challenge: In all of the reactions, mixtures and buffers mentioned so far, be able to identify the weak acid or weak base and its conjugate that forms the salt. Challenge: The addition of the corresponding salt to an existing WA or WB equilibrium is known as “the common ion effect”. What affect doesthe added salt have on the WA or WB equilibria? For a weak acid buffer comprised of acetic acid and sodium acetate: HA (aq) H + (aq) + A - (aq) acetic acid sodium acetate For a weak base buffer comprised of ammonia and ammonia chloride: B (aq) OH - (aq) + BH + (aq) ammonia ammonium chloride Challenge : Why isn’t the counter ion of the salt shown in theseequlibria? Challenge : Qualitatively, how doesthe pH of a buffer solution compare to the parent weak acid or base? How would you calculate the pH of a buffer? Use the following example verify your predictions. CU C1131 – Baker RAP Buffers and Titrations Dr. Christine Kelly Modified 1.25.07 Page 2 of 19
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Example: A buffer solution was prepared by mixing 0.50M acetic acid and 0.50M sodium acetate. What is the pH of this solution? How doesthis pH compare to the pH of a 0.50M acetic acid solution of 2.52? Solution: Acetic acid (aq)  H + (aq) + acetate anion (aq) I 0.50 0 0 + 0.50 (salt) C E K a = 1.8x10 -5 = (_________)(_________) /(__________) Solve for x via the approximation: Check with the 5% dissociation rule : Solve for the pH: Comparison: 0.50MAcetic acid Acetic acid solution * buffer pH 2.52 4.75 % dissociation 0.60% 0.0036% * Calculated in a separate ICE table calculation. Practice : What is the pH of a buffer that consists of 1.5 M ammonia and 2.5 M in ammonium chloride? K b for ammonia is 1.8x10 -5 . CU C1131 – Baker RAP Buffers and Titrations Dr. Christine Kelly Modified 1.25.07 Page 3 of 19
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C. Henderson-Hasslebalch Equation pH = pKa + log - [A ] [HA] pOH = pK b + log + [BH ] [B] Challenge : Can you see how the Henderson-Hasselbalch is the result of an ICE table calculation? Derive the Henderson-Hasselbalch Equation.
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