Exam 3-solutions

# Exam 3-solutions - Version 330 – Exam 3 – mccord...

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Unformatted text preview: Version 330 – 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 high pressures. 3. high pressures and low molar masses. 4. high temperatures and low pressures. correct 5. low temperatures and low 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 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. 18.7 mol 2. 20.5 mol 3. 7.8 mol 4. 15.7 mol 5. 9.3 mol correct 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... P 2 /P 1 = n 2 /n 1 20 . 7 / 31 . 0 = n 2 / 28 . n 2 = 18 . 7 moles Note that is 18.7 moles of gas still in the tank. The amount used is therefore... 28 .- 18 . 7 = 9 . 3 mols O 2 003 4.0 points A sample of gas at 200 ◦ C has a volume of 5 L. The volume expands to 10 L due to a temperature change at constant pressure. What will be the new temperature? 1.- 36 . 5 ◦ C 2. 673 ◦ C correct 3. ◦ C 4. 400 ◦ C 5. 100 ◦ C Explanation: V 1 = 5 L V 2 = 10 L T 1 = 200 ◦ C + 273 = 473 K Applying Charles’ Law, V 1 T 1 = V 2 T 2 T 2 = V 2 T 1 V 1 = (10 L) (473 K) 5 L = 946 K- 273 = 673 ◦ C 004 4.0 points At 20 ◦ C the vapor pressure of dry ice is 56.5 atm. If 32 g of dry ice (solid CO 2 ) is placed in an evacuated 0.25 L chamber at a constant temperature of 20 ◦ C, will all of the solid dry ice sublime? (why?) 1. Yes. (because the vapor pressure is much greater than 1 atm) Version 330 – Exam 3 – mccord – (51600) 2 2. No. (because the temperature isn’t low enough) 3. Yes. (because the dry ice cannot exist at 20 ◦ C) 4. No. (because the vapor pressure will be reached before all the dry ice is gone) correct Explanation: The dry ice will sublime until the pressure is equal to the vapor pressure. There are 0.73 moles (32/44) of CO 2 . If all 0.73 moles sublime the pressure will be 70 atm which is MORE than the vapor pressure. This means that the solid with only sublime up to the pressure of 56.5 atm and that there will still be solid present. To be exact, 25.8 g of dry ice will sublime (exact amount to reach the 56.5 atm), and 6.2 g will still be solid....
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Exam 3-solutions - Version 330 – Exam 3 – mccord...

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