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Unformatted text preview: Version 380 – Exam 3 – mccord – (51600) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 4.0 points Real gases behave most nearly like ideal gases at 1. low temperatures and high pressures. 2. high temperatures and low pressures. correct 3. low temperatures and low pressures. 4. high pressures and low molar masses. 5. high temperatures and high pressures. Explanation: At high temperatures the gas molecules are moving more rapidly and the effects of the attractive forces are less significant. At low pressures the molecules are on average much further apart and the effects of the attractive forces are less significant because there are fewer ‘close encounters’. 002 4.0 points Consider four molecules I) CHCl 3 II) CH 4 III) CH 3 Cl IV) CCl 4 Which of these exhibit permanent dipole- dipole interactions? 1. I only 2. I, III, and IV only 3. III only 4. I and III only correct 5. None of these Explanation: Dipole-dipole interactions occur in polar molecules. CHCl 3 and CH 3 Cl are polar be- cause their dipole moments do not cancel. CH 4 and CCl 4 are symmetric; their dipole moments cancel and the overall molecule is non-polar. 003 4.0 points 2.00 moles of a gas occupies 62.2 L at a par- ticular temperature and pressure. Its density is 0.900 g/L. The standard molar volume is 22.4 L/mole. What is the molar mass? 1. 34.6 g/mol 2. Cannot be determined 3. 12.4 g/mol 4. 28.0 g/mol correct 5. 10.1 g/mol Explanation: n = 2 . 00 mol V = 62 . 2 L density = 0 . 900 g / L molar volume = 22 . 4 L / mole Take density times volume to get weight. Then divide weight by moles to get molar mass. That is, parenleftbigg . 900 g L parenrightbiggparenleftbigg 62 . 2 L 2 . 00 mol parenrightbigg = 28.0 g/mol. 004 4.0 points A 500 mL sample of Liquid A at 25 ◦ C takes 5 minutes to flow through a narrow-necked funnel. A 500 mL sample of liquid B at 25 ◦ C takes 7 minutes to flow through the same sized funnel. Which of these liquids would you predict to have the highest surface tension? 1. liquid A 2. liquid B correct 3. They have the same surface tension. Explanation: Liquid B has the higher viscosity since it took longer to flow through the funnel. This means it probably has stronger IM forces than liquid A, thus a higher surface tension. Version 380 – Exam 3 – mccord – (51600) 2 005 4.0 points An oxygen tank kept at 20.0 ◦ C contains 28.0 moles of oxygen and the gauge reads 31.0 atm. After two weeks, the gauge reads 20.7 atm. How much oxygen was used during the two-week period? 1. 20.5 mol 2. 9.3 mol correct 3. 18.7 mol 4. 7.8 mol 5. 15.7 mol Explanation: Pressure change is due to mole change. This is similar do Charles’ Law except that pres- sure is directly proportional to moles (keeping volume and temperature constant). So that......
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- Spring '07