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Unformatted text preview: Version 304 Exam 3 mccord (50970) 1 This printout should have 30 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. McCord CH301 This exam is only for Dr. McCords CH301 classes. 001 10.0 points A mixture of CO, CO 2 and O 2 is contained within a 275 mL flask at 0 C. If the total pres sure is 780 torr, the CO has a partial pressure of 330 torr and the CO 2 has a partial pressure of 330 torr, what is the partial pressure of O 2 ? 1. 900 torr 2. 780 torr 3. 120 torr correct 4. 330 torr 5. 660 torr Explanation: P total = 780 torr P CO = 330 torr P CO 2 = 330 torr P total = P CO + P CO 2 + P O 2 P O 2 = P total P CO P CO 2 = 780 torr 330 torr 330 torr = 120 torr 002 10.0 points Which gas would you expect to have the largest value for the van der Waals constant a? 1. CH 4 2. Ne 3. NH 3 correct 4. He Explanation: 003 10.0 points The gauge in this problem is an absolute pres sure gauge so assume the pressure is exactly as stated in this problem (do not adjust it). An oxygen tank kept at 20 C contains 28 moles of oxygen and the gauge reads 31 atm. After two weeks, the gauge reads 10 . 5 atm. How much oxygen was used during the two week period? 1. 9.25 mol 2. 9.48 mol 3. 18 . 52 mol correct 4. 7.51 mol 5. 20.5 mol Explanation: 28 mol 9 . 48 mol = 18 . 52 mol 004 10.0 points Nitric acid is produced commercially by the Ostwald process. In the first step, ammonia is oxidized to nitric oxide: 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) Assume this reaction is carried out in the apparatus diagramed below. 2.00 L NH 3 0.500 atm 1.00 L O 2 1.50 atm The stopcock between the two reaction con tainers is opened, and the reaction proceeds using proper catalysts. Calculate the partial pressure of NO after the reaction is complete. Assume 100% yield for the reaction, assume Version 304 Exam 3 mccord (50970) 2 the final container volume is 3.00 L, and as sume the temperature is constant. 1. 0.250 atm 2. 0.333 atm correct 3. 0.500 atm 4. 1.50 atm 5. 0.400 atm Explanation: First, calculate the partial pressures of NH 3 and O 2 after the stopcock is opened (and before the reaction). Use Boyles Law... new volume is 3 L so that... P NH 3 = (2L/3L)(1/2atm) = 1/3 atm P O 2 = (1L/3L)(3/2atm) = 1/2 atm You must have at least 5/4 more moles of O 2 than NH 3 for complete reaction. That means you need 5/12 atm of O 2 . You have more than enough (0.5 atm = 6/12 atm) so the oxygen is in excess. The easy part... You make the SAME num ber of moles of NO as you use NH 3 , so the final partial pressure of the NO will be the same as the initial partial pressure of the NH 3 which is 1/3 atm or 0.333 atm....
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This note was uploaded on 11/07/2010 for the course CH 301 taught by Professor Fakhreddine/lyon during the Fall '07 term at University of Texas at Austin.
 Fall '07
 Fakhreddine/Lyon
 Chemistry

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