CHAPTER 15 CONCEPTUAL QUESTIONS (PART II)

CHAPTER 15 CONCEPTUAL QUESTIONS (PART II) - CHAPTER 15...

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CHAPTER 15 CONCEPTUAL QUESTIONS (PART II) BUFFER CONCEPTS
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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. C. For the reasons stated in “B”, “C” is true. D. The buffer will be just as resistant in its reaction to strong acid or base, as long as the buffer system is not overwhelmed with strong acid or base. “D” is false. E. The pH will only equal 7 if the K a of the acid and conjugate base were equal, i.e., 1 x 10 -7 . Several of the answers can also be determined quantitatively: HCN + H 2 O
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15 CHEM 162-2006 HOURLY EXAM III + ANSWERS CHAPTER 15 – APPLICATIONS OF AQUEOUS EQUILIBRIA BUFFER CONCEPTS What is the net ionic equation, when a solution of potassium hydroxide is added to a solution of nitrous acid. A. HNO 2 + K + + OH - → KNO 2 + H 2 O B. HNO 2 + H 2 O → NO 2 - + H 3 O + C. HNO 2 + KOH → K + + NO 2 - + H 2 O D. HNO 2 + OH - → NO 2 - + H 2 O E. H + + OH - → H 2 O Complete equation: HNO 2 + KOH HOH + KNO 2 Ionic equation: HNO 2 + K + + OH - HOH + K + + NO 2 - Net ionic equation: HNO 2 + OH - HOH + NO 2 -
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19. CHEM162-2000 HOURLY EXAM II BUFFER CONCEPTS Which one of the following statements concerning buffers is false? A. A buffer can consist of a weak base and a salt containing its conjugate acid in the same solution. B. A buffer changes pH only slightly when small amounts of strong acid or strong base are added. C. The pH of a buffer solution should be within one pH unit of the pK a of the acid component of the buffer. D. The pH of a buffer will not change significantly if the buffer is diluted. E.
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CHAPTER 15 CONCEPTUAL QUESTIONS (PART II) - CHAPTER 15...

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