ch 302hw 8 pretest solution

ch 302hw 8 pretest solution - reyes (mr29667) H08:...

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Unformatted text preview: reyes (mr29667) H08: pre-exam3 Mccord (90540) 1 This print-out should have 16 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which one of the following combinations is NOT a buffer solution? 1. CH 3 COOH and NaCH 3 COO 2. HCN and NaCN 3. HBr and KBr correct 4. NH 3 and NH 4 Br 5. NH 3 and (NH 4 ) 2 SO 4 Explanation: A buffer must contain a weak acid/base conjugate pair. HBr/Br- is a strong acid conjugate pair. CH 3 COOH / CH 3 COO- , HCN / CN- and NH + 4 / NH 3 are weak acid/base conjugate pairs. 002 10.0 points What is the pH of a solution which is 0.600 M in dimethylamine ((CH 3 ) 2 NH) and 0.400 M in dimethylamine hydrochloride ( (CH 3 ) 2 NH + 2 Cl- ) ? K b for dimethylamine = 7 . 4 10- 4 . 1. 3.31 2. 10.87 3. 11.21 4. 10.78 5. 11.05 correct 6. 10.69 7. 2.95 Explanation: K w = 1 10- 14 K a = 0 . 00074 [(CH 3 ) 2 NH] = 0.6 M [(CH 3 ) 2 NH + 2 ] = 0.4 M K a , (CH 3 ) 2 NH + 2 = K w K b , (CH 3 ) 2 NH Applying the Henderson-Hasselbalch equa- tion, pH = p K a + log parenleftbigg [(CH 3 ) 2 NH] [(CH 3 ) 2 NH + 2 ] parenrightbigg = log parenleftbigg K w K a parenrightbigg + log parenleftbigg [(CH 3 ) 2 NH] [(CH 3 ) 2 NH + 2 ] parenrightbigg = log parenleftbigg 1 10- 14 . 00074 parenrightbigg + log parenleftbigg . 6 . 4 parenrightbigg = 11 . 0453 003 10.0 points A solution of an unknown weak acid (HA) has a ratio [A- ] [HA] = 1 2000 at a pH of 3. What would be this ratio at a pH of 5? 1. 20 1 2. 2000 3. 1 20 correct 4. 3 5 5. 10- 3 2 Explanation: pH ini = 3.00 pH fin = 5.00 [A- ] [HA] = 1 2000 pH ini = p K a + log parenleftBigg bracketleftbig A- bracketrightbig [HA] parenrightBigg p K a = pH ini log parenleftBigg bracketleftbig A- bracketrightbig [HA] parenrightBigg = 3 log parenleftbigg 1 2000 parenrightbigg log parenleftbigg [A- ] [HA] parenrightbigg = pH fin p K a = 5 bracketleftbigg 3 log parenleftbigg 1 2000 parenrightbiggbracketrightbigg reyes (mr29667) H08: pre-exam3 Mccord (90540) 2 = 1 . 30103 bracketleftbig A- bracketrightbig...
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This note was uploaded on 08/07/2010 for the course CH 90540 taught by Professor Mccord during the Summer '10 term at University of Texas at Austin.

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ch 302hw 8 pretest solution - reyes (mr29667) H08:...

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