7 - Elbel Brittany Homework 7 Due Mar 6 2006 noon Inst J A...

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Elbel, Brittany – Homework 7 – Due: Mar 6 2006, noon – Inst: J A Holcombe 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. 001 (part 1 of 1) 10 points What is the H + ion concentration in a 0.50 mol/L solution of a weak base that has an ionization constant ( K b ) of 2.0 × 10 - 8 ? 1. 1 . 0 × 10 - 4 mol/L 2. 2 . 0 × 10 - 10 mol/L 3. 8 . 0 × 10 - 16 mol/L 4. 1 . 0 × 10 - 10 mol/L correct 5. 1 . 0 × 10 - 8 mol/L Explanation: 002 (part 1 of 1) 10 points Hydroxylamine is a weak molecular base with K b = 6 . 6 × 10 - 9 . What is the pH of a 0.0500 M solution of hydroxylamine? 1. pH = 9.26 correct 2. pH = 10.37 3. pH = 7.12 4. pH = 3.63 5. pH = 8.93 6. pH = 9.48 7. pH = 4.74 Explanation: Hydroxylamine is a weak base, so use the equation to calculate weak base [OH - ] con- centration (note that this is the approximate equation. Why? Because K b is very small and the concentration is reasonable) : [OH - ] = p K b C b = q (6 . 6 × 10 - 9 ) (0 . 0500) = 1 . 82 × 10 - 5 After finding [OH - ], you can find pH using either method below: A) pOH = - log ( 1 . 82 × 10 - 5 ) = 4 . 74 pH = 14 - 4 . 74 = 9 . 26 or B) [H + ] = K w [OH - ] = 1 . 0 × 10 - 14 1 . 82 × 10 - 5 = 5 . 52 × 10 - 10 pH = - log ( 5 . 52 × 10 - 10 ) = 9 . 26 003 (part 1 of 1) 10 points A 0.200 M solution of a weak monoprotic acid HA is found to have a pH of 3.00 at room temperature. What is the ionization constant of this acid? 1. 2.0 × 10 - 9 2. 5.0 × 10 - 3 3. 5.30 4. 1.8 × 10 - 5 5. 1.0 × 10 - 6 6. 2.0 × 10 - 5 7. 5.0 × 10 - 6 correct 8. 1.0 × 10 - 3 Explanation: 004 (part 1 of 1) 10 points Consider the acid ionization constants
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Elbel, Brittany – Homework 7 – Due: Mar 6 2006, noon – Inst: J A Holcombe 2 Ionization Acid Constant K a value HF 7 . 2 × 10 - 4 HNO 2 4 . 5 × 10 - 4 CH 3 COOH 1 . 8 × 10 - 5 HClO 3 . 5 × 10 - 8 HCN 4 . 0 × 10 - 10 Which is the weakest acid? 1. HCN correct 2. CH 3 COOH 3. HNO 2 4. HF 5. HClO Explanation: 005 (part 1 of 1) 10 points Assume that five weak acids, identified only by numbers (1, 2, 3, 4 and 5), have the follow- ing ionization constants. Ionization Acid Constant K a value 1 1 . 0 × 10 - 3 2 3 . 0 × 10 - 5 3 2 . 6 × 10 - 7 4 4 . 0 × 10 - 9 5 7 . 3 × 10 - 11 The anion of which acid is the weakest base? 1. 2 2. 1 correct 3. 4 4. 5 5. 3 Explanation: HA * ) H + + A - K a = [H + ] [A - ] [H] [A] The ‘anion of an acid’ is another way of saying ‘conjugate base,’ and a weak conjugate base corresponds to a strong acid. So really what we’re looking for is which acid is strongest (has the lowest pH). A low pH means that the [H + ] concentra- tion is low. (Remember that values greater than 7 are basic!) The larger values of K a means that there is more [H + ] so you would expect these solutions to be more acidic; i.e., have smaller pH’s. The smaller K a values mean less [H + ] in solution, so higher pH’s.
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