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209final_2007answers

209final_2007answers - CHEMISTRY 209 FINAL EXAM Your Name(P...

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CHEMISTRY 209 FINAL EXAM December 10, 2007 Your Name: ( Please print ) Last First CORNELL NetID Day and Time of Your Lab-Recitation Section: ANSWERS Complete academic integrity is expected of all students of Cornell University at all times, whether in the presence or absence of members of the faculty or teaching staff. Understanding this, I declare that I shall not give, use, or receive unauthorized aid in this examination. Your Signature: There are 7 (many of them multipart) questions and a total of 15 pages (including this cover sheet, a blank page after all the questions, a periodic table and a page of potentially useful facts at the end). Make sure that your examination booklet is complete. The final is not designed with the idea that all, or necessarily any of you, will complete each and every question; rather, it provides a menu covering many different areas of relevance to the course, and I expect that each of you should have something substantial to say about most of those areas. Write your answers directly on these sheets. Use the blank page, or if necessary the backs of pages, but clearly indicate on the problem page where work can be found. If you need to include additional pages, please attach them securely to the rest of the test book. Be brief, concise, and neat. Include only material pertinent to the question asked. Proctors are not allowed to interpret questions for you. In case of doubt, indicate the assumptions on which your answer is based. Pay attention that the significant figures reported in your answers are not unreasonable. You may leave when you are finished, but please do so quietly after turning your paper. Question 1. (25) Question 2. (20) Question 3. (20) Question 4. (20) Question 5. (25) Question 6. (20) Question 7. (20) Total (150) Show your work! (Unless you don’t care to receive partial credit). 1
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1. (a) (9 points) Consider three identical flasks filled with different gases: Flask A: CO at 760 torr and 0 C Flask B: N 2 at 250 torr and 0 C Flask C: H 2 at 100 torr and 0 C i. In which flask will the molecules have the greatest average kinetic energy? Average kinetic energy is determined by temperature, and nothing else. So all flasks have the same average kinetic energy at the same temperature. ii. In which flask will the molecules have the greatest root mean square velocity? Average velocity, however defined, is proportional to the square root of the quotient of temperature divided by mass. At fixed T, the fastest molecules are the lightest–Flask C. iii. In which flask will there be the greatest frequency of collisions between gas molecules and the wall of the flask? Collision frequency depends on (1) how many molecules are in the flask, or pressure (at fixed temperature) and (2) the velocity of the molecules. Flask A has the most molecules; Flask C, the fastest. But the number of molecules in flask A is 7.6 times the number in C; each molecule travels at speed which on average is 2 28 1 3 . 8 more slowly; still, there are more collisions with the wall in Flask A.
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