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Exam 3 - McCord

# Exam 3 - McCord - Version 304 Exam 3 mccord(50970 This...

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Version 304 – Exam 3 – mccord – (50970) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH301 This exam is only for Dr. McCord’s 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

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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 Boyle’s 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. 005 10.0 points The density of the vapor of allicin, a compo- nent of garlic, is 1 . 14 g · L 1 at 125 C and 175 Torr. What is the molar mass of allicin? 1. 162 g · mol 1 correct 2. 21 . 6 g · mol 1 3. 50 . 8 g · mol 1 4. 869 g · mol 1 5. 273 g · mol 1 Explanation: T = 125 C + 273.15 K = 398 . 15 K P = (175 Torr) 1 atm 760 Torr = 0 . 230263 atm ρ = 1 . 14 g / L The ideal gas law is P V = n R T n V = P R T with unit of measure mol/L on each side.
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