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Exam #1 Solutions

# Exam #1 Solutions - Version 134 Exam 1 mccord(51620 This...

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Version 134 – Exam 1 – mccord – (51620) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCordCH302 1pm class only - 51620 For water: k f = 1 . 86 C / m k b = 0 . 51 C / m C s , ice = 2 . 09 J/g C C s , water = 4 . 18 J/g C C s , steam = 2 . 03 J/g C Δ H fus = 334J / g Δ H vap = 2260J / g 001 10.0points CarbonDioxide 3 1 4 2 A B 200 250 300 350 400 10 0 10 1 10 2 10 3 10 4 Pressure, bar Temperature, K A sample of carbon dioxide is stored at 10 4 bar and 250 K. This sample is then decompressed to 10 0 bar at constant temper- ature. Then, at constant pressure it is heated to 400 K. Next, it is compressed at con- stant temperature to 200 bar. According to the phase diagram, how many phase transi- tions has the carbon dioxide gone through, and what is its final state? 1. 3, supercritical fluid 2. 3, gas 3. 2, supercritical fluid correct 4. 2, gas 5. 2, liquid Explanation: Navigate through the diagram through each process. In the first decompression, the solid is decompressed and becomes a liquid and then a gas (2 phase transitions). Upon heating, it becomes a supercritical fluid (no phase transitions). 002 0.0points If more points are awarded on this assign- ment, would you like them added to your score? 1. YES, I would like the points and the higher score. correct 2. NO, leave my score alone, I prefer the lower score Explanation: This should be a no-brainer. Most students want higher scores. If you picked yes, you got credit for the question and you got the extra points you asked for (if they were granted by your instructor). If you answered NO, you also got what you wanted... no points awarded. 003 10.0points A semi-permeable membrane can withstand an osmotic pressure of 0 . 75 atm. What mo- larity of aqueous magnesium bromide solution would reach the limit for this membrane? (as- sume RT = 25 L · atm · mol 1 ). 1. 0 . 01 M correct 2. 1 M 3. 0 . 03 M

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Version 134 – Exam 1 – mccord – (51620) 2 4. 0 . 03 mM 5. 0 . 01 mM Explanation: Use the formula for osmotic pressure. Re- member that magnesium bromide will disso- ciate into 3 particles, so there will be a Van’t Hoff factor of 3 for this calculation. 004 10.0points An unknown liquid has a vapor pressure of 88 mmHg at 45 C and a heat of vaporization of 32 kJ / mol, what is its vapor pressure at 55 C? 1. 61 mmHg 2. 127 mmHg correct 3. 89 . 4 mmHg 4. 156 mmHg Explanation: Use the Clausius-Clapeyron Equation. Here, the only thing we don’t know is P 2 . ln parenleftbigg P 2 P 1 parenrightbigg = Δ H vap R parenleftbigg 1 T 1 - 1 T 2 parenrightbigg ln parenleftbigg P 2 88 parenrightbigg = 32 , 000 8 . 31 parenleftbigg 1 318 - 1 328 parenrightbigg ln P 2 = 32 , 000 8 . 31 parenleftbigg 1 318 - 1 328 parenrightbigg + ln 88 P 2 = e 4 . 84 = 127 mmHg 005 10.0points Which of the following statements about col- ligative properties of aqueous solutions is FALSE? 1. Osmosis is a colligative property.
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Exam #1 Solutions - Version 134 Exam 1 mccord(51620 This...

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