Practice Exam III ANSWERS - SP09

# Practice Exam III ANSWERS - SP09 - April 2009 1 PRACTICE...

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April, 2009 1 PRACTICE EXAM III – ANSWERS 1. (a) What is the conjugate acid of the hydroxide anion? H 2 O (b) What is the conjugate base of water? OH (c) Complete the following acid-base (proton transfer) reactions. Write “acid” or “base” below each species and “link together” the conjugate acid-base pairs, as shown in class: (i) NH 4 + (aq) + HCO 3 (aq) NH 3 (aq) + H 2 CO 3 (aq) Acid Base Base Acid ---------------------------------- ----------------------------------- (ii) CH 3 O (aq) + NH 3 (aq) CH 3 OH(aq) + NH 2 (aq) Base Acid Acid Base ----------------------------------- ---------------------------------- (d) Using the table of relative strengths of acids and bases (Fig 15.2, p. 692, which will be provided), will K c for equilibrium (c)(i) be greater or less than 1? Explain. Less than 1. H 2 CO 3 is a stronger acid than NH 4 + , and NH 3 is a stronger base than HCO 3 , so the equilibrium will tend to go to the left; a higher concentration of reactants than products so K c < 1 2. Place the letter corresponding to your answer on the blank line to the right of each question. (a) Which of the following is not a strong base? A. Ba(OH) 2 B. Al(OH) 3 C. LiOH D. KOH B (b) Which substance listed below produces a basic solution when dissolved in pure water? A. NH 3 B. NaBr C. NH 4 I D. KNO 3 A (c) Which substance listed above in question (b) produces an acidic solution when dissolved in pure water? C (d) In a buffer solution, the ratio [base]/[acid] should be no greater than. .. A. 1.0 B. 0.1 C. 10.0 D. 0.01 C (e) To prepare an acidic buffer solution, its pH must be: A. equal to the pK a of the acid B. greater than the pK a of the acid B. less than the pK a of the acid D. within ±1 pH unit of the pK a

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April, 2009 2 3. You are asked to prepare 100.00 mL of a basic buffer solution of pH = 10.00. Using the information below, answer the following: C 6 H 5 NH 2 K b = 4.3 x 10 –10 pK b = 9.37 pK a = 4.63 (pK a + pK b = pK w = 14.00) N 2 H 4 K b = 8.9 x 10 –7 pK b = 6.05 pK a = 7.95 CH 3 NH 2 K b = 3.7 x 10 –4 pK b = 3.43 pK a = 10.57 (a) Which base would you select? Explain why. You would select CH 3 NH 2 since its pK a is within 1 pH unit of the pH of the buffer. (b) What will be the formula of the conjugate acid that you would also need to use? Remember, for every cation you need an anion! CH 3 NH 3 + Cl (or another anion that is neutral) (c) If the concentration of the base is 0.100 M, what concentration of acid is needed? Using pH = pK
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Practice Exam III ANSWERS - SP09 - April 2009 1 PRACTICE...

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