Chapter 19-Buffers

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 change over time in these systems? 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 substance listed below, give the other substance necessary 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 volumes of 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 volumes of 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 does the 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 these equlibria? Challenge : Qualitatively, how does the 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 does this 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
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This note was uploaded on 04/01/2008 for the course CHEM 1131 taught by Professor Redin,kend during the Summer '08 term at Colorado.

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

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