exam--3 - Version 169 Exam 3 Sutcliffe (52410) 1 This...

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Unformatted text preview: Version 169 Exam 3 Sutcliffe (52410) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. NOTE: The values of K a for acetic acid and K b for ammonia may be found in a different question; they are used in several questions but not quoted in all of them. 001 10.0 points A buffer (pH 9.25) was prepared by mixing 1.00 mole of ammonia and 1.00 mole of am- monium chloride to form an aqueous solution with a total volume of 1.00 liter. To 500 ml of this solution was added 30.0 ml of l.00 M NaOH. What is the pH of this solution? 1. 8.96 2. 9.83 3. 9.11 4. 9.31 correct 5. 9.57 Explanation: Given: [NH + 4 ] = 1 M [NaOH] = 1 M [NH 3 ] = 1 M pH ini = 9.25 Initial condition (ini): n NaOH = 30 . 1 . 0 = 30 mmol n Na + = 0 n NH 4 + = 500 1 . 0 = 500 mmol n NH 3 = 500 1 . 0 = 500 mmol NaOH + NH + 4 Na + + NH 3 + H 2 O ini 30 500 500 30 30 30 30 fin 470 30 530 Na + is a spectator ion. NH + 4 / NH 3 is a buffer system. Since [NH 3 ] = [NH + 4 ] in the original buffer p K a = pH ini = 9 . 25, and pH fin = p K a + log parenleftBigg [NH 3 ] bracketleftbig NH + 4 bracketrightbig parenrightBigg = 9 . 25 + log parenleftbigg 530 470 parenrightbigg = 9 . 30218 002 10.0 points What is the approximate pH of a solution in which the concentration of nitrous acid is 0 . 2 M and the concentration of sodium nitrite is . 9 M? K a for nitrous acid is 4 . 5 10 4 . 1. 2 . 64 2. 1 . 53 3. 6 . 3 4. 5 . 65 5. 4 correct Explanation: [NO 2 ] = 0 . 9 M K a = 4 . 5 10 4 [HNO 2 ] = 0 . 2 M HNO 2 H + + NO 2 pH = p K a + log parenleftbigg [NO 2 ] [HNO 2 ] parenrightbigg = log(4 . 5 10 4 ) + log parenleftbigg . 9 . 2 parenrightbigg = 4 003 10.0 points What is the pH of a 0 . 2 M solution of Ca(OH) 2 ? Assume complete dissociation of the base. 1. 10.8 2. 7 3. 13.6 correct 4. 0.4 Explanation: [Ca(OH) 2 ] = 0.2 M Ca(OH) 2 is a strong base which completely dissociates. Ca(OH) 2 Ca 2+ + 2 OH [OH ] = 2 [Ca(OH) 2 ] = 2 (0 . 2 M) = 0 . 4 M pH = 14 pOH = 14 log 0 . 4 = 13 . 6 Version 169 Exam 3 Sutcliffe (52410) 2 004 10.0 points What is the pH of an aqueous solution that is 0.60 M (CH 3 ) 3 N ( K b = 6 . 5 10 5 ) and 0.95 M (CH 3 ) 3 NHCl? 1. 4.39 2. 9.61 correct 3. 10.01 4. 9.81 5. 3.99 Explanation: 005 10.0 points When a small amount of NaOH is added to a solution of 0.10 M NH 3 and 0.10 M NH 4 Cl, 1. Nothing happens. 2. [OH ] decreases slightly. 3. the K b for NH 3 becomes smaller. 4. the OH ions react with NH + 4 ions. cor- rect 5. the pH decreases slightly. Explanation: [NH 3 ] = 0.l0 M [NH 4 Cl] = 0.10 M This is an NH + 4 / NH 3 buffer system. Most of the added OH (from NaOH) would react with NH + 4 , shifting equilibrium to the left, and pH would increase only slightly from the small increase of [OH ]. K b remains constant....
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exam--3 - Version 169 Exam 3 Sutcliffe (52410) 1 This...

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