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

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

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CHAPTER 15 – 2 nd HALF APPLIC. OF ACID & BASE EQUILIBRIA (BUFFERS & TITRATIONS) 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. The pH depends on the K a vs the K cb . The Ka is 6.2 x 10 -10 . The K cb is (1 x 10 -14 )/(6.2 x 10 -10 ) = 1.61 x 10 -5 . Therefore, the acid is a weaker acid in formation of hydronium ions as compared to the strength of the base in the formation of hydroxide ions. Therefore, more hydroxide ions will form than hydronium ions, so the pH will be above 7. “B” is false.
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