Homework #4 Solutions - purdy (sbp456) Homework 4 owers...

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purdy (sbp456) – Homework 4 – fowers – (53885) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – Fnd all choices be±ore answering. HW 4. Due Tuesday 07 October 2008 11 PM. 001 (part 1 o± 2) 10.0 points A 0 . 73 g sample o± sodium hydrogen sulFte (NaHSO 3 ) is dissolved in 54 mL o± water. What is the pH o± the solution? p K a 1 = 1 . 81 and p K a 2 = 6 . 91. 1. 6.45 2. None o± these 3. 8.14 4. 5.64 5. 7.0 6. 4.36 correct Explanation: p K a 1 = 1 . 81 p K a 2 = 6 . 91. This is a salt o± a polyprotic acid which will dissociate in solution. The cation is an extremely weak acid and will not a²ect the equilibrium. HSO 3 can act as both an acid and a base. Both actions need to be consid- ered simultaneously: HSO 3 + H 2 O H 3 O + + SO 2 3 HSO 3 + H 2 O H 2 SO 3 + OH and The extent to which these processes occur will be determined by the relative values o± p K a 1 and p K a 2 . To Fnd pH we use the ±ollowing: pH = 1 2 (p K a 1 + p K a 2 ) = 1 2 (1 . 81 + 6 . 91) = 4 . 36 . Note that the pH is independent o± the con- centration o± the anion as long as it is not extremely dilute. At extremely low concentrations o± HSO 3 , however, the approximations upon which the use o± this equation are based are no longer valid. 002 (part 2 o± 2) 10.0 points What is the pH o± a solution resulting ±rom the dissolution o± 210 mg o± AgNO 3 in 5 mL o± water? 1. 5.64 2. 6.45 3. 7.0 correct 4. 4.36 5. None o± these 6. 8.14 Explanation: Neither the silver ion nor the nitrate ion is acidic or basic in aqueous solution, so the pH is o± the neutral water: 7. 003 10.0 points Adding NH 4 IO to pure water would result in what kind o± solution? K a ±or HIO is 2 . 3 × 10 11 and K b ±or NH 3 is 1 . 8 × 10 5 . 1. neutral 2. basic correct 3. acidic Explanation: 004 10.0 points Which o± the ±ollowing is a basic salt? 1. BaCl 2 2. AlBr 3 3. Al(OH) 3 4. Ca(OH)Cl correct Explanation:
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purdy (sbp456) – Homework 4 – fowers – (53885) 2 A basic salt contains a cation other than hydrogen, and an anion other than oxide or hydroxide. 005 10.0 points Adding NH 4 NO 2 to pure water would result in what kind oF solution? K a For HNO 2 is 4 . 5 × 10 4 and K b For NH 3 is 1 . 8 × 10 5 . 1. basic 2. acidic correct 3. neutral Explanation: 006 (part 1 oF 2) 10.0 points An aqueous solution has a dilute HNO 3 con- centration oF 1 . 0 × 10 10 M at 298 K. What is the greatest contributor toward solution pH? 1. extend oF ionization oF the acid HNO 3 2. autoionization oF water correct 3. temperature 4. concentration oF the acid HNO 3 Explanation: The strong acid is 10 3 times more dilute than the H + in solution due to the water (solvent). The pH depends mostly on the autoionization oF water in this case.
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This note was uploaded on 11/06/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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Homework #4 Solutions - purdy (sbp456) Homework 4 owers...

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