exam 3 - Monteiro Dalton Exam 3 Due Nov 8 2007 11:00 pm...

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Monteiro, Dalton – Exam 3 – Due: Nov 8 2007, 11:00 pm – Inst: McCord 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. McCord 11 AM ONLY ! ! ! ! 001 (part 1 of 1) 10 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. 9.3 mol correct 3. 7.8 mol 4. 20.5 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... P 2 /P 1 = n 2 /n 1 20 . 7 / 31 . 0 = n 2 / 28 . 0 n 2 = 18 . 7 moles Note that is 18.7 moles of gas still in the tank. The amount used is therefore... 28 . 0 - 18 . 7 = 9 . 3 mols O 2 002 (part 1 of 1) 10 points Generally if a liquid has stronger intermolec- ular attractions it will have A. a higher viscosity; B. a higher vapor pressure; C. a higher boiling point. 1. A and C only correct 2. B only 3. A, B, and C 4. B and C only 5. A only 6. A and B only 7. None of the properties 8. C only Explanation: The stronger intermolecular forces will lead to higher viscosity and a higher boiling point. The high boiling point is due to a lower vapor pressure. 003 (part 1 of 1) 10 points Surface tension describes 1. the forces of attraction between sur- face molecules of a solvent and the solute molecules. 2. the forces of attraction between the sur- face of a liquid and the air above it. 3. the inward forces that must be overcome in order to expand the surface area of a liquid. correct 4. the resistance to flow of a liquid. 5. capillary action. 6. adhesive forces between molecules. Explanation: Molecules in the interior of a liquid inter- act with molecules all around them, whereas molecules at the surface of a liquid can only

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Monteiro, Dalton – Exam 3 – Due: Nov 8 2007, 11:00 pm – Inst: McCord 2 be affected by those beneath the surface layer. This phenomenon leads to a net inward force of attraction on the surface molecules, con- tracting the surface and making the liquid behave as though it had a skin. Surface ten- sion is a measure of the inward forces that must be overcome to expand the surface area of a liquid. 004 (part 1 of 1) 10 points Calculate the pressure of 4 g of nitrogen gas in a 1 L container at 20.0 C. 1. 0.235 atm 2. 0.469 atm 3. 6.87 atm 4. 3.44 atm correct 5. 96.2 atm Explanation: m = 4 g V = 1 L T = 20 C + 273.15 = 293.15 K For the nitrogen, n = (4 g N 2 ) · 1 mol N 2 28 . 0134 g N 2 = 0 . 142789 mol N 2 P V = n R T P = n R T V = (0 . 142789 mol N 2 ) ( 0 . 08206 L · atm mol · K ) 1 L × (293 . 15 K) = 3 . 43491 atm 005 (part 1 of 1) 10 points Dipole-dipole forces are 1. about a factor of two smaller than the attraction of ± 1 ions. 2. much smaller than ion-ion forces. cor- rect 3. about the same as all ion-ion forces.
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