q3ps11key

# q3ps11key - CH302 Spring 2011 Practice Quiz 3 Answer KeyThe...

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CH302 Spring 2011 Practice Quiz 3 Answer Key—The TA Version 1. Which of the following pairs of solutions would result in a buffer upon mixing? 2 2 SO 4 3. 100 mL of 1 M NH 3 3 4. 150 mL of 3 M Ba(OH) 2 & 200 mL of 2 M HClO 5. 100 mL of 1 M CH 3 COOH & 50 mL of 1 M NaOH Correct Explanation: 100 mL of 1 M CH 3 COOH & 50 mL of NaOH, would equate to 0.1 mol of a weak acid and 0.05 mol of a strong base. Upon neutralization, the resulting solution would contain 0.05 mol each of acetic acid and its conjugate base, acetate - resulting in a buffered system. 2. What would be the pH of a solution prepared from 2 L of H 2 O, 85 g of NH 3 and 98 g of NH 4 Br? Assume the K b of ammonia is 2x10 -5 . 1. 4 2. 5.4 3. 10 Correct 4. 8.6 5. 7 Explanation: 85 g of NH 3 x 1 mol/17 g = 5 mol NH 3 5 mol NH 3 / 2 L H 2 O = 2.5 M NH 3 98 g of NH 4 Br x 1 mol/98 g = 1 mol NH 4 Br 1 mol NH 4 Br / 2 L H 2 O = 0.5 M NH 4 Br [OH - ] = K b (C b \C a ) = 2x10 -5 (2.5 M/0.5 M) = 10 -4 pH = 10 3. Two liters of a buffer containing 0.6 M CH 3 NH 2

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q3ps11key - CH302 Spring 2011 Practice Quiz 3 Answer KeyThe...

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