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Unformatted text preview: amg2845 – Exam 1 – McCord – (53580) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 1 atm = 760 torr 1 atm = 101325 Pa 1 bird/hand = 2 bird/bush 001 10.0 points Estimate the enthalpy of vaporization of CCl 4 given that at 25 ◦ C and 58 ◦ C its vapor pressure is 107 and 405 torr, respectively. Assume that the enthalpy of vaporization is independent of the temperature. 1. 486 J · mol − 1 2. 3.98 kJ · mol − 1 3. 48.6 kJ · mol − 1 4. 33.1 kJ · mol − 1 correct 5. 142 kJ · mol − 1 Explanation: 002 10.0 points Note: Assume all solutions are prepared using the same solvent. Which of the following has the lowest freez- ing point and the highest boiling point? 1. 1.0 m sodium chloride 2. 1.5 m calcium chloride 3. 2.0 m potassium chloride 4. 1.5 m aluminum nitrate 5. 1.5 m magnesium phosphate correct Explanation: 003 10.0 points The vapor pressure of a liquid at 25 ◦ C is 200 torr. What will happen when the barometric pressure, P bar , is lowered to 190 torr? 1. The liquid will begin to sublime. 2. Δ H f for the liquid will approach Δ G vap . 3. The liquid will begin to boil. correct 4. More information is needed to determine what will happen. 5. The vapor pressure of the liquid will abruptly increase to 1 atm because the so- lution is boiling. Explanation: When the barometric pressure is decreased below the vapor pressure of the liquid, it will begin to boil. Sublimation occurs when the phase transition is from solid to gas. The vapor pressure will not increase to 1 atm (760 torr). 004 10.0 points Solution A contains 1 mole of NaBr dissolved in 1000 grams of water and solution B con- tains 1 mole of sugar dissolved in 1000 grams of water. Which of the following is false? 1. The vapor pressure of solution B is about 55 . 5 56 . 5 times that of pure water. 2. The boiling point of solution A is higher than that of solution B. 3. The vapor pressure of solution A is higher than that of solution B. correct 4. The freezing point of solution A is lower than that of solution B. Explanation: m A , H 2 O = 1000 g n A , NaBr = 1 mol m B , H 2 O = 1000 g n B , sugar = 1 mol Sucrose is a non-electrolyte which does not dissociate in water but sodium bromide is a strong electrolyte which dissociates as follows: NaBr → Na + + Br − Therefore, an equal concentration of aque- amg2845 – Exam 1 – McCord – (53580) 2 ous sodium chloride (A) will have twice the colligative effect of aqueous sucrose (B). The colligative properties are vapor pressure low- ering, freezing point depression, boiling point elevation and osmotic pressure. P soln B P water = x water P water P water = x water x water = P soln B P water = mol H 2 O mol H 2 O + mol NaBr = 1000 18 1000 18 + 1 = 55 . 5 56 . 5 005 10.0 points The weak acid HZ (175 g/mol) will ionize somewhat when dissolved in water. 150 mL of solution contains 0 . 82 g of the acid. This solution has an osmotic pressure of 775 torr at...
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This test prep was uploaded on 03/19/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.
- Spring '07