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Unformatted text preview: Version 247 – Exam 1 – Mccord – (52450) 1 This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points At 20 ◦ C the vapor pressure of dry ice is 56.5 atm. If 10 g of dry ice (solid CO 2 ) is placed in an evacuated 0.25 L chamber at a con- stant temperature of 20 ◦ C, will all of the solid sublime? 1. Yes correct 2. Only if it is at the triple point 3. No Explanation: The dry ice will sublime until the pressure is equal to the vapor pressure. There are 0.227 moles of CO 2 . If all sublimes the pressure will be less than the vapor pressure, so the solid will never reach equilibrium with the gas, and the system will be all gas. 002 10.0 points The vapor pressure of pure acetone (CH 3 COCH 3 ) at 30 o C is 240 torr. If salt (NaCl) is dissolve into a sample of acetone until the total vapor pressure of the mixture at 30 o C is 180 torr, what is the mol fraction of acetone? 1. 0.75 correct 2. 0.2 3. 0.25 4. 0.8 Explanation: χ water + χ acetone = 1 χ acetone = x , χ water = 1- x P = P o water · χ water + P o acetone · χ acetone 190 torr = 40 torr · (1- x) + 240 torr · x x = 0 . 75 003 10.0 points An animal cell assumes its normal volume when it is placed in a solution with a total solute molarity of 0.3 M. If the cell is placed in a solution with a total solute molarity of 0.1 M, 1. water enters the cell, causing expansion. correct 2. the escaping tendency of water in the cell increases. 3. water leaves the cell, causing contrac- tion. 4. no movement of water takes place. Explanation: 004 10.0 points What is a solution containing all of the so- lute that a solvent can dissolve at a certain temperature and pressure? 1. saturated correct 2. unsaturated 3. concentrated 4. supersaturated Explanation: 005 10.0 points The vapor pressure of all liquids 1. increases with volume of liquid present. 2. is the same at 100 ◦ C. 3. is the same at their freezing points. 4. increases with temperature. correct 5. decreases with the increasing volume of the container. Explanation: As temperature (kinetic energy) increases, rate of evaporization increases and rate of con- Version 247 – Exam 1 – Mccord – (52450) 2 densation decreases; therefore, vapor pressure will increase with increasing temperature. 006 10.0 points On the CO 2 phase diagram the slope of the solid-liquid equilibrium is positive. This is because 1. the density of solid CO 2 is higher than liquid CO 2 . correct 2. CO 2 is a non-polar molecule. 3. the enthalpy of vaporization for CO 2 is positive. 4. the density of liquid CO 2 decreases with temperature. 5. the triple point of CO 2 is higher than one atmosphere....
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- Spring '07
- Chemistry, Vapor pressure, Phase transition