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# exam 1 - mj6727 Exam 1 McCord(53580 This print-out should...

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mj6727 – 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 The osmotic pressure of 1.00 g of a polymer dissolved in benzene to give 200 mL of solu- tion is 1.50 kPa at 25 C. Estimate the average molar mass of the polymer. The gas law con- stant is 0.0821 L · atm · mol 1 · K 1 . 1. 693 g · mol 1 2. 62,000 g · mol 1 3. 1650 g · mol 1 4. 41,300 g · mol 1 5. 8260 g · mol 1 correct Explanation: 002 10.0 points Note: The question is asking for the CHANGE in vapor pressure. What is the change in vapor pressure from that of pure water for an aqueous solution at 40 C in which the mole fraction of fructose is 0 . 33? The vapor pressure of pure water at 40 C is 55 . 34 torr. 1. 12 . 78 torr 2. 21 . 91 torr 3. 18 . 26 torr correct 4. 20 . 09 torr 5. 16 . 43 torr 6. 14 . 61 torr Explanation: P pure solvent = 55 . 34 torr T = 40 χ fructose = 0 . 33 χ water = 1 - 0 . 33 = 0 . 67 P = χ solvent P pure solvent = 0 . 67 (55 . 34 torr) = 37 . 0778 torr . The change in vapor pressure is Δ P = 55 . 34 torr - 37 . 0778 torr = 18 . 26 torr . 003 10.0 points Compared to the diameter of a water molecule, the diameter of a colloid is 1. 10 to 10,000 times larger. correct 2. about the same. 3. more than 25,000 times larger. 4. 2 to 4 times larger. Explanation: Colloids particles are macromolecules or a conglomeration of ions. 004 10.0 points Δ G vap for H 2 O( ) at 85 C is ( < 0, = 0, > 0) and at 100 C is ( < 0, = 0, > 0). 1. > 0; = 0 correct 2. < 0; > 0 3. < 0; < 0 4. > 0; > 0 5. < 0; = 0 Explanation: 005 10.0 points Which one of the following statements is NOT correct?

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mj6727 – Exam 1 – McCord – (53580) 2 1. When Δ G for a reaction is negative the reaction can occur spontaneously. 2. When Δ H for a reaction is positive the re- action cannot occur spontaneously. correct 3. When Δ G for a reaction is positive the reaction cannot occur spontaneously. 4. When Δ G for a reaction is zero the system is at equilibrium. 5. When Δ H for a reaction is negative the reaction can occur spontaneously. Explanation: When Δ G is negative, the process is spon- taneous. When Δ G = 0, the process is at equilibrium. When Δ G is positive, the pro- cess is nonspontaneous. The sign of Δ H has no general definite effect on spontaneity since Δ G = Δ H - T Δ S . Δ G is dependent on Δ H and T Δ S . 006 10.0 points Methanol (CH 3 OH) and ethanol (C 2 H 5 OH) are common organic solvents. The vapor pres- sures of pure methanol and pure ethanol at 25 C p m s and 58 . 9 torr, respectively. If the mole fraction of CH 3 OH in the mixture of the two solvents at 25 C is 0 . 35, what is the mole fraction of C 2 H 5 OH in the vapor phase that is in equilibrium with the liquid mixture? 1. 0 . 556 2. 0 . 404 3. 0 . 641 4. 0 . 241 5. 0 . 321 6. 0 . 471 correct 7. 0 . 161 Explanation: P methanol = 122 . 7 torr χ methanol = 0 . 35 P ethanol = 58 . 9 torr χ ethanol = 1 - χ methanol = 1 - 0 . 35 = 0 . 65 .
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exam 1 - mj6727 Exam 1 McCord(53580 This print-out should...

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