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Unformatted text preview: Version 101 Exam 3 Mccord (52385) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which of the following cations is likely to be hydrated in compounds? 1. La 3+ correct 2. K + 3. Cs + 4. Rb + 5. NH + 4 Explanation: La 3+ has the highest charge. 002 10.0 points You were given the values 0 . 2107, 2 . 253, 3 . 392, and 17 . 58 as the van der Waals parameter a for the gases CO 2 , CH 3 CN, Ne, and CH 4 but NOT the correspondence of the values to the compounds. What a value shoud be assigned to Ne? 1. 3 . 392 L 2 atm / mol 2. . 2107 L 2 atm / mol correct 3. Unable to determine 4. 2 . 253 L 2 atm / mol 5. 17 . 58 L 2 atm / mol Explanation: Since the van der Waals parameter a rep- resents the role of attractions, it will become larger as the strength of the intermolecular attractions increases. Acetonitrile is polar and would be expected to have the strongest intermolecular forces and the highest value of a . The three non-polar species can be ranked in order of the polarizability of their elec- tron clouds. Carbon dioxide has the largest, most polarizable electron cloud, so it should have the strongest London dispersion forces of these three species and the highest value of a after acetonitrile. Methane would be next, while neon would exhibit the weakest London dispersion forces and the lowest value of a . We can check this reasoning by first noting that acetonitrile is the only substance that is a liquid at room temperature. In fact, the normal boiling points of these chemical substances, as given in the third column of the table, fall in the same order as their van der Waals a parameters. a Substance B.P. ( C) 17.58 CH 3 CN 81 . 6 3.392 CO 2- 78 . 5 (subl.) 2.253 CH 4- 164 . 76 0.2107 Ne- 246 . 048 003 10.0 points 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. 7.51 mol 2. 9.25 mol 3. 20.5 mol 4. 18 . 52 mol correct 5. 9.48 mol Explanation: 28 mol- 9 . 48 mol = 18 . 52 mol 004 10.0 points Which molecules of the following gases will have the greatest average kinetic energy? 1. CO 2 at 1 atm and 298 K 2. H 2 at 0.5 atm and 298 K 3. All of the molecules have the same kinetic Version 101 Exam 3 Mccord (52385) 2 energy. correct 4. N 2 at 1 atm and 298 K 5. He at 0.1 atm and 298 K Explanation: From kinetic molecular theory, the temper- ature is directly proportional to mean KE. If all of the gases are at the same tempera- ture they have the same average KE. (Note: The average speeds will be different since they have different molecular weights.) 005 10.0 points Which of the following gases would you pre- dict to have the largest value of the van der Waals coefficient b ?...
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- Spring '08