Chem 162-2011 Chapter 15B-Some Application of acid and base equilibria practice problems

Chem 162-2011 - CHAPTER 15 2nd HALF APPLIC OF ACID BASE EQUILIBRIA(BUFFERS TITRATIONS BUFFER CONCEPTS 1 10 Chem 162-2007 Final exam answers Chapter

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CHAPTER 15 – 2 nd HALF BUFFER CONCEPTS 1
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10 Chem 162-2007 Final exam + answers Chapter 15B – Applications of Acid & Base Equilibria Buffer concepts Which of the following solutions would be a buffer solution? A. 50 mL 1 M HCl + 50 mL 1 M NH 3 B. 50 mL 1 M HCl + 50 mL 1 M NaOH C . 25 mL 1 M NaOH + 50 mL 1 M CH 3 COOH D. 25 mL 1 M NaOH + 50 mL 1 M NH 3 E. 50 mL 1 M NaOH + 25 mL 1 M CH 3 COOH o The concentrations of the acids and bases are each diluted for “Initial” values. o A buffer must be a combination of a weak acid and its conjugate base, or a weak base and its conjugate acid. A. NH 3 + H 3 O + NH 4 + + H 2 O Since this contains a strong acid, bring the reaction to completion. NH 3 + H 3 O + NH 4 + + H 2 O Initial 0.5 0.5 0 Change -0.5 -0.5 +0.5 Equilibrium 0 0 +0.5 Since the equilibrium solution only contains a conjugate acid (i.e., no weak base), this isn’t a buffer. B. H 3 O + + OH - 2H 2 O Since this contains a strong acid (as well as a strong base), bring the reaction to completion. H 3 O + + OH - 2H 2 O Initial 0.5 0.5 0 Change -0.5 -0.5 +1.0 Equilibrium 0 0 +1.0 The equilibrium solution contains no buffer components. C. HA + OH - H 2 O + A - Since this contains a strong base, bring the reaction to completion. HA + OH - H 2 O + A - Initial 0.67 0.33 0 0 Change -0.33 -0.33 +0.33 Equilibrium 0.33 0 0.33 Since the equilibrium solution contains a weak acid and a conjugate base, this is a buffer. D. NH 3 + OH - → No reaction Since the equilibrium solution contains only a weak base and a strong base, this is not a buffer. E. HA + OH - H 2 O + A - Since this contains a strong base, bring the reaction to completion. HA + OH - H 2 O + A - Initial 0.33 0.67 0 0 Change -0.33 -0.33 +0.33 Equilibrium 0 0.33 0.33 Since the equilibrium solution contains a conjugate base but no weak acid, this is not a buffer. 2
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7 CHEM 162-2006 HOURLY EXAM III + ANSWERS CHAPTER 15 – APPLICATIONS OF AQUEOUS EQUILIBRIA BUFFER CONCEPTS Consider a potential buffer solution containing 2.0 M HCN and 1.0 M NaCN (K a for HCN = 6.2 × 10 -10 ). Which one of the following statements is true? A. The solution is not a buffer, because [HCN] is not equal to [CN - ]. B. The pH will be below 7.00, because the concentration of the acid is greater than that of the base. C . [OH - ] > [H + ] D. The buffer will be more resistant to pH changes from addition of strong acid than of strong base. E. The pH will equal 7.00, because the acid is very weak. A. This is a weak acid and its conjugate base in a 1:2 ratio. A buffer can be a weak acid and its conjugate base in a 1:10 ratio to a 10:1 ratio. Hence, this is a buffer. “A” is false. B.
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This document was uploaded on 11/02/2011 for the course GEN CHEM 162 at Rutgers.

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Chem 162-2011 - CHAPTER 15 2nd HALF APPLIC OF ACID BASE EQUILIBRIA(BUFFERS TITRATIONS BUFFER CONCEPTS 1 10 Chem 162-2007 Final exam answers Chapter

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