Exam 3 - Version 128 – Exam 3 – mccord –(50970 1 This...

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Unformatted text preview: Version 128 – 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. 333 Torr 2. 299 Torr 3. 446 Torr 4. 688 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 As we increase the temperature of a liquid, its properties change. Which of the following would NOT be an expected change in the properties of a typical liquid as we increase its temperature? 1. increase in tendency to evaporate 2. decrease in density 3. decrease in viscosity 4. increase in vapor pressure 5. increase in surface tension correct Explanation: As temperature increases, molecules have more energy to overcome intermolecular forces/bonds. Therefore viscosity, surface tension and density will decrease and vapor pressure and rate of evaporation will increase. 003 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 ? 1. 330 torr 2. 780 torr 3. 660 torr 4. 120 torr correct 5. 900 torr Explanation: Version 128 – Exam 3 – mccord – (50970) 2 P total = 780 torr P CO = 330 torr P CO 2 = 330 torr P total = P CO + P CO 2 + P O 2 P O 2 = P total- P CO- P CO 2 = 780 torr- 330 torr- 330 torr = 120 torr 004 10.0 points A gas has a volume of 2.00 liters at a tempera- ture of 127 ◦ C. What will be the volume of the gas if the temperature is increased to 327 ◦ C? (Assume the pressure remains constant.) 1. 6.00 liters 2. 4.00 liters 3. 2.00 liters 4. 3.00 liters correct Explanation: T 1 = 127 ◦ C + 273 = 400 K V 1 = 2 . 00 L T 2 = 327 ◦ C + 273 = 600 K Charles’ Law relates the volume and ab- solute (Kelvin) temperature of a sample of gas: V 1 T 1 = V 2 T 2 V 2 = V 1 T 2 T 1 = (2 . 00 L) (600 K) 400 K = 3 . 00 L 005 10.0 points Which of the following can form intermolecu- lar hydrogen bonds? 1. PH 3 2. CH 3 COCH 3 3. (CH 3 ) 2 NH correct 4. H 2 CO Explanation: Only molecules with H attached to the electronegative atoms N, O, and F can hy- drogen bond. Of the molecules given only (CH 3 ) 2 NH 2 has the H directly bonded to N, so it can undergo hydrogen bonding....
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This note was uploaded on 02/10/2011 for the course CH 301 taught by Professor Fakhreddine/lyon during the Fall '07 term at University of Texas.

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Exam 3 - Version 128 – Exam 3 – mccord –(50970 1 This...

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