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Exam3_Solutions - Version 130 Exam 3 McCord(53740 This...

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Version 130 – Exam 3 – McCord – (53740) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Make sure you are taking the right exam. This is Dr. McCord’s MORNING course that is MWF from 11:00 - 12 noon, unique 53740. McCord MWF : 11am g = 9 . 8 m / s 2 1 atm = 101325 Pa = 760 torr R = see back of answer sheet P = ρ · g · h ( P + a ( n/V ) 2 )( V - nb ) = nRT 001 10.0 points If the average speed of a water molecule at 25 C is 640 m · s 1 , what is the average speed at 100 C? 1. 801 m/s 2. 320 m/s 3. 572 m/s 4. 716 m/s correct 5. 5120 m/s Explanation: T 1 = 100 C + 273 . 15 = 373 . 15 K T 2 = 25 C + 273 . 15 = 298 . 15 K From kinetic molecular theory, the temper- ature is directly proportional to mean KE. KE mean = 1 2 (MW)(average molecular speed) 2 and knowing MW is constant (it’s the same gas) we get average speed of gas molecules Avg speed T 1 Avg speed T 2 = T 1 T 2 Avg speed T 2 = (Avg speed T 1 ) T 2 T 1 = (640 m / s) 373 . 15 k 298 . 15 K = 715 . 986 m / s i.e. , average speed of gas molecules at 100 C is 715 . 986 m / s. 002 10.0 points Which of the following gases would you pre- dict to have the largest value of the van der Waals coefficient b ? 1. CO 2 2. C 2 FCl 5 correct 3. C 2 F 6 4. Cl 2 5. C 2 F 2 Cl 4 Explanation: The larger the van der Waals coefficient ‘ b ’, the larger the molecule. The largest molecules have 8 atoms. Of these, C 2 FCl 5 is larger than C 2 F 2 Cl 4 and C 2 F 6 , because Cl is larger than F. 003 10.0 points Which would you expect to be least viscous? 1. C 4 H 8 at 30 C 2. C 8 H 18 at 50 C 3. C 8 H 18 at 30 C 4. C 4 H 8 at 50 C correct Explanation: 004 10.0 points Which of the following can form intermolecu- lar hydrogen bonds? 1. PH 3 2. H 2 CO 3. (CH 3 ) 2 NH correct
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Version 130 – Exam 3 – McCord – (53740) 2 4. CH 3 COCH 3 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 (CH 3 ) 2 NH 2 has the H directly bonded to N, so it can undergo hydrogen bonding. 005 10.0 points If 250 mL of a gas at STP weighs 2 g, what is the molar mass of the gas? 1. 56 . 0 g · mol 1 2. 44 . 8 g · mol 1 3. 179 g · mol 1 correct 4. 28 . 0 g · mol 1 5. 8 . 00 g · mol 1 Explanation: V = 250 mL P = 1 atm T = 0 C = 273.15 K m = 2 g The density of the sample is ρ = m V = 2 g 0 . 25 L = 8 g / L The ideal gas law is P V = n R T n V = P R T with unit of measure mol/L on each side. Multiplying each by molar mass (MM) gives n V · MM = P R T · MM = ρ , with units of g/L. MM = ρ R T P = (8 g / L)(0 . 08206 L · atm / mol / K) 1 atm × (273 . 15 K) = 179 . 318 g / mol 006 10.0 points If the average speed of a carbon dioxide molecule is 410 m · s 1 at 25 C, what is the average speed of a molecule of methane at the same temperature? 1. 679 m · s 1 correct 2. 1000 m · s 1 3. 410 m · s 1 4. 1130 m · s 1 5. 247 m · s 1 Explanation: From kinetic molecular theory, the temper- ature is directly proportional to mean KE.
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