10-17-11 - Chemistry 1A F11 10.17.11 Dr Shaka Exam...

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112 Chemistry 1A F11 10.17.11 Dr. Shaka Course Code 40000 Exam Postmortem Many mistakes! Units conversions, no units, and arithmetic errors topped the list. Assigned problems, which we went over in the Discussion Sections, were often missed. Apparently very few could determine the amount of molecular iodine in the gas. Also the sodium/strontium carbonate problem proved difficult for many. The van der Waals problems were mostly okay.
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113 Chemistry 1A F11 10.17.11 Dr. Shaka Course Code 40000 Exam Postmortem An example. How many ozone molecules are present in one cubic meter of air at 250 K and 1.3 × 10 3 atm? (1 m = 100 cm, 1 L = 1000 cm 3 ). n = pV RT = 1.0 " 10 # 3 atm ( ) 1 m 3 100 cm / m ( ) 3 1 l /1000 cm 3 ( ) 0.0820575 latm mol # 1 K # 1 ( ) 250 K = 0.0487463 moles $ N = N 0 n = 6.0221415 " 10 23 0.0487463 ( ) = 2.93557 " 10 22 molecules N.B. If 1 m = 100 cm, (1m) 3 = (100 cm) 3 , and not 100 cm 3 . Always put parentheses around the number and the units .
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114 Chemistry 1A F11 10.17.11 Dr. Shaka Course Code 40000 Exam Postmortem Another example. If 0.0457 moles of I 2 is put into a 2.30 liter container at 1410 K, what would the pressure be if all the gas were I 2 ? This rounds to 2.30 atm to three digits. But putting 2.29(89) is also completely acceptable. We cannot assume that any “simple rules” for sig. figs. will actually work in all situations. p = nRT V = 0.0457 mol " 0.0820575 latm mol # 1 K # 1 " 1410 K 2.30 l = 2.29893 atm
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115 Chemistry 1A F11 10.17.11 Dr. Shaka Course Code 40000 Exam Postmortem Now supposing all the I 2 converted to I atoms according to I 2 ( g ) = 2I( g )? Well, there are just twice as many moles now, and the temperature and volume are the same, so it must be that p = 4.60 atm. No calculation required!
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