9_new - Homework #7 holcombe (51160) This print-out should...

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Homework #7 – holcombe – (51160) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. K b = 10 −pK B = 10 −4.75 = 1.77828 × 10 −5 Building an ICE using molarities, 001 10.0 points Which of the following is NOT true for a solution bu f ered at a selected pH? 1. The bu f ered solution could be made to be either acidic or basic. 2. The bu f ered solution contains an acid and its conjugate base. 3. The solution pH will not change with the addition of an acid. correct 4. The solution pH will increase a lesser amount than a non-bu f ered solution, when base is added. Explanation: Bu f ers are solutions which contain compar-ative amounts of a conjugate weak acid/base part and allow for only small changes in the pH of a chemical system, compared to non-bu f ered solutions, when an acid or a base is introduced. 002 10.0 points What is the pH of a solution containing 0.3 M NH 4 Cl and 0.6 M NH 3 ? The pK a of the ammonium ion is 9.25. 1. 9.55 correct 2. 5.05 3. 12.25 4. 8.95 5. 4.45 Explanation: For NH + 4 , pK a = 9.25 pK b = pK w − pK a = 14 − 9.25 = 4.75 pK b = − log K b + NH3(aq)+H2O(ℓ) NH4 (aq)+OH (aq) 0.6 −x +x +x 0.6 − x 0.3 + x +x + K b = [NH 4 ] [OH ] = (0.3 + x) x 0.3 x [NH 3 ] 0.6 − x 0.6 x = 0 .6 K b = 0.6 (1.77828 × 10 −5 ) 0.3 0.3 = 3.55656 × 10 −5 = [OH ] [H + ] = K w = 1 × 10 −14 [OH ] 3.55656 × 10 −5 = 2.81171 × 10 −10 Thus pH = − log(2.81171 × 10 −10 ) = 9.55103 003 10.0 points What is the pH of an aqueous solution that is 0.10 M HCOOH (K a = 1.8 × 10 −4 ) and 0.10 M NaHCO 2 ? 1. 2.38 2. 5.74 3. 10.26 4. 5.62 5. 3.74 correct Explanation: 004 10.0 points
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Homework #7 – holcombe – (51160) 2 Which of the following combinations CAN- NOT produce a bu f er solution? All compo- nents are present in 0.10 M concentration. 1. NH 3 and (NH 4 ) 2 SO 4 2. KF and HF 3. HBr and NaBr correct 4. HNO 2 and NaNO 2 5. HClO and NaClO Explanation: All are weak conjugate acid/base pairs ex-cept for HBr/Br which is a strong conjugate acid/base pair. 005
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This note was uploaded on 04/04/2012 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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9_new - Homework #7 holcombe (51160) This print-out should...

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