CHE2 - Version 039 Exam 2 mccord (51620) 1 This print-out...

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Unformatted text preview: Version 039 Exam 2 mccord (51620) 1 This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. McCord - CH302 1pm class only - 51620 001 10.0 points What is [H 3 O + ] in a solution made by dissolving 0.100 mole of sodium acetate (NaCH 3 COO) in enough water to make one liter of solution? K a for CH 3 COOH is 1 . 80 10 5 . 1. 8.87 2. 1 . 80 10 6 3. 5.74 4. 5 . 56 10 11 5. 7 . 46 10 6 6. 5 . 56 10 10 7. 9.25 8. 5.13 9. 10.25 10. 1 . 34 10 9 correct Explanation: 002 10.0 points The equilibrium constant K c for the reaction 2 NOCl(g) 2 NO(g) + Cl 2 (g) is 0.51 at a certain temperature. A mixture of NOCl, NO, and Cl 2 with concentrations 1.3, 1.2, and 0.60 M, respectively, was introduced into a container at this temperature. Which of the following is true? 1. Cl 2 (g) is produced until equilibrium is reached. 2. [NOCl] = [NO] = [Cl 2 ] at equilibrium. 3. [Cl 2 ] = 0.30 M at equilibrium. 4. No apparent reaction takes place. cor- rect 5. NOCl(g) is produced until equilibrium is reached. Explanation: 003 10.0 points The plain K given in this problem is actually K c . K = 8, for the gas phase reaction N 2 O 4 2 NO 2 . If 2 moles of N 2 O 4 are put into a 1 liter vessel, the number of moles of NO 2 at equilib- rium ( x ) can be calculated from the equation 1. 8 = 2 x 2 2- x 2. 8 = x 2 2- 1 2 x correct 3. 8 = x 2 2 4. 8 = x 2- 1 2 x 5. 8 = x 2 2- x Explanation: Since the problem asks you to put the equa- tion in terms of x , which we are told is equal to [NO 2 ], we need a slight modification to our normal way of working this problem. Working this problem the normal way tells us that fi- nal [NO 2 ] is equal to 2 x . Continuing with this method will not give us the correct answer. Normal way: Version 039 Exam 2 mccord (51620) 2 N 2 O 4 2 NO 2 (g) Initial 2 Change- x +2 x Equilibrium 2- x 2 x In the modified way, all x coefficients are divided in half to give the appropriate [NO 2 ] = x equality. This is ok to do because weve kept the correct ratio in our change step; i.e. , [NO 2 ] must be twice as big as [N 2 O 4 ]. Modified way: N 2 O 4 2 NO 2 (g) Initial 2 Change- 1 2 x + x Equilibrium 2- 1 2 x x Continuing with the modified way . . . K = 8 = [NO 2 ] 2 [N 2 O 2 ] = x 2 2- 1 2 x 004 10.0 points Which of the statements concerning equilib- rium is NOT true? 1. The value of the equilibrium constant for a given reaction is the same regardless of the direction from which equilibrium was attained. 2. A system moves spontaneously toward a state of equilibrium. 3. Equilibrium in molecular systems is dy- namic, with two opposing processes balancing one another....
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This note was uploaded on 03/22/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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CHE2 - Version 039 Exam 2 mccord (51620) 1 This print-out...

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