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Exam 3-solutions

# Exam 3-solutions - Version 395 Exam 3 mccord(50970 This...

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Version 395 – 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 oxygen and helium is 92.3% by mass oxygen. It is collected at atmospheric pressure (687 torr). What is the partial pressure of oxygen in this mixture? 1. 688 Torr 2. 333 Torr 3. 299 Torr 4. 446 Torr 5. 414 Torr correct Explanation: Assume you have 100 g of this mixture; calculate the number of moles: n O 2 = (92 . 3 g O 2 ) 1 mol O 2 31 . 9988 g O 2 = 2 . 91261 mol O 2 . n He = (7 . 7 g He) 1 mol He 4 . 0026 g He = 1 . 92375 mol He . n tot = n O 2 + n He = 2 . 91261 mol O 2 + 1 . 92375 mol He = 4 . 83636 mol gas Dalton’s Law: P O 2 = P tot × χ O 2 = P tot × n O 2 n tot = (687 Torr) 2 . 91261 mol O 2 4 . 83636 mol gas = 413 . 733 Torr 002 10.0 points A sample of helium effuses through a porous container 5.80 times faster than does un- known gas X. What is the molar mass of the unknown gas? 1. 282.24 2. 125.44 3. 144.0 4. 100.0 5. 134.56 6. 225.0 7. 268.96 8. 345.96 9. 207.36 10. 331.24 Correct answer: 134 . 56 g / mol. Explanation: effusion rate He = 5 . 8 × effusion rate of gas X MW X = ? rate of effusion of He rate of effusion of X = MW X MW He radicalbig MW X = parenleftbigg rate of effusion of He rate of effusion of X parenrightbigg radicalbig MW He = parenleftbigg 5 . 80 1 parenrightbigg radicalbig 4.00 g/mol = 11 . 6 radicalbig g / mol MW X = parenleftBig 11 . 6 radicalbig g / mol parenrightBig 2 = 134 . 56 g / mol 003 10.0 points Which of the following gases would you pre- dict to have the largest value of the van der Waals coefficient b ? 1. C 2 F 2 Cl 4

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Version 395 – Exam 3 – mccord – (50970) 2 2. C 2 FCl 5 correct 3. C 2 F 6 4. CO 2 5. Cl 2 Explanation: The larger the van der Waals coefficient b , the larger the molecule. The largest molecules have 8 atoms. Of these, C 2 FCl 5 is larger than C 2 F 2 Cl 4 and C 2 F 6 , because Cl is larger than F. 004 10.0 points Consider the following statements. Which are true? I) Vapor pressure is an equilibrium phe- nomenon. II) The smaller the IMF, the smaller the vapor pressure. III) The volume of a liquid does not affect the vapor pressure IV) Vapor pressure is temperature depen- dent. 1. I, II, and III 2. III, and IV 3. I, III, and IV correct 4. I, and III 5. I, II, and IV 6. I, II, III, and IV 7. II, III, and IV Explanation: As vapor particles encounter the surface of a solution and condense, their energy released makes another particle move as a vapor. This is an equilibrium process, and it is tempera- ture dependent. As IMF decreases, the va- por pressure increases, since only weak forces will be holding a substance together as a liq- uid. Vapor pressure also only occurs at the surface, so it does not matter how deep the liquid phase is. 005 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 ?
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