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Exam4_Solutions_Alternative

Exam4_Solutions_Alternative - Version 070 Exam 4...

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Version 070 – Exam 4 – Sutcliffe – (52410) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. SPECIAL NOTE: DUE TO THE LOG- ARITHMIC NATURE OF SOME OFTHE EQUATIONS YOU MAY FIND IF YOU DO NOT USE THE EXACT CONVER- SION TO KELVIN YOUR VALUE MAY BE SLIGHTLY DIFFERENT TO THE AN- SWER. IT SHOULD BE VERY CLOSE. 001 10.0 points When we titrate a weak base with a strong acid, the pH at the equivalence point will be 1. pH > 7. 2. pH = 7. 3. pH < 7. correct Explanation: At the equivalence point, all of the strong acid will be consumed, leaving only its very weak (to the point of being neutral) conjugate base behind. However, the weak base will be converted into its corresponding conjugate acid, which will not be neutral. The resulting solution will then be acidic , having a pH < 7. 002 10.0 points NOTE: A strong complex is one in which ev- erything is very tightly bound together. An- other way to put this would be to say that K d for FeEDTA is VERY tiny. Fe(OH) 3 (s) is very insoluble in water ( K sp = 1 . 6 × 10 39 ; however, Fe 3+ forms a strong complex with EDTA (FeEDTA , K f = 1 . 3 × 10 25 ). For a solution which is at equilibrium and contains Fe(OH) 3 (s) precipi- tate, which of the following occurs if EDTA is added to the solution? 1. More Fe(OH) 3 (s) precipitates because [Fe 3+ ] decreases. 2. More Fe(OH) 3 (s) dissolves. correct 3. Nothing happens because K sp is much smaller than K f . 4. No more Fe(OH) 3 (s) dissolves, but Fe 3+ complexes with EDTA. Explanation: 003 10.0 points The rate constant for a reaction is 1 . 50 × 10 8 s 1 at 0 C and has an activation energy of 45 kJ · mol 1 . What is the predicted rate constant at 200 C? 1. 1 . 51 × 10 8 s 1 2. 6 . 5 × 10 5 s 1 correct 3. 3 . 1 × 10 6 s 1 4. 68.4 s 1 5. 7 . 4 × 10 11 s 1 Explanation: 004 10.0 points The un-ionized form of an acid indicator is yel- low and its anion is blue. The K a of this indi- cator is 10 6 . What will be the approximate pH range over which this indicator changes color? 1. 4 6 2. 9 11 3. 3 5 4. 5 7 correct 5. 8 10 Explanation: K a = 10 6 The p K a of this indicator is 6, so the indica- tor will change color around pH 6. Thus you would expect a color change between a pH of 5 and 7. 005 10.0 points NOTE: Use a=1 for this question
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Version 070 – Exam 4 – Sutcliffe – (52410) 2 For the reaction cyclopropane(g) propene(g) at 500 C, a plot of ln[cyclopropane] vs t gives a straight line with a slope of 0 . 00067 s 1 . What is the order of this reaction and what is the rate constant? 1. second order; 6 . 7 M 1 · s 1 2. second order; 6 . 7 × 10 2 M 1 · s 1 3. first order; 6 . 7 s 1 4. second order; 6 . 7 × 10 4 M 1 · s 1 5. first order; 6 . 7 × 10 4 s 1 correct 6. first order; 6 . 7 × 10 2 s 1 7. None of these Explanation: 006 10.0 points Note the ionization constant given is K b .
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