Exam II - SP09 - Version A - key

Exam II - SP09 - Version A - key - 1 Name: _ CHEMISTRY 1032...

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1 Name: _____________________________ CHEMISTRY 1032 EXAM II – VERSION A Name __________________________________ TUID ____________________ (print clearly) Show all of your work where requested. No graphing or memory accessible calculators are allowed at your examination desk. No cells phones or PDAs are allowed on your persons. Violation of these department rules will be brought to the attention of the Chemistry Department Chairman, the Dean and the Provost, and may result in significant disciplinary action for violation of university standards of academic integrity. Read the problems carefully. Give numerical answers to the correct number of significant figures. The numbers in the [ ] at the end of each question are the point values. Check your work! Useful information is found on page 11.
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2 Name: _____________________________ 1. A pharmaceutical company is preparing to market a new antibiotic, Owlmycin A. But before they can do this, they must comply with the FDA requirement to establish a “shelf life” for the product. That is, they must determine how long the Owlmycin A can remain on the pharmacy shelf and still be safe to use. The company has decided that the product can remain on the shelf as long as 95% of the original dose is in the tablet. To determine this time, they will perform a series of experiments at higher temperatures so that k for the first order decomposition of Owlmycin A at 25 o C can be estimated. Rates used to prepare the plot were in hr –1 . [20] Arrhenius Plot for Decomposition of Owlmycin A y = -9996.2x + 19.852 R 2 = 0.9985 -12 -10 -8 -6 -4 -2 0 0.002 0.0022 0.0024 0.0026 0.0028 0.003 0.0032 (a) What is plotted on the x-axis? [1] 1/time (b) What is plotted on the y-axis? ln(k) [1] (c) Use the Arrhenius plot to determine the value of the pre-exponential factor, A, to three significant figures. What are the units of A? [3] intercept = 19.852 = lnA A = e^(19.852) = 4.18 x 10^8 hr -1 (d) Use the Arrhenius plot to calculate the value of the energy of activation, in kJ/mol, to three significant figures, for the decomposition of Owlmycin A. [3] -E a /R = - 9996.2 E a = 8.314 x 9996.2 x 1 kJ/(1000 J) = 83.1 kJ/mole (e) Calculate k for the reaction at 25 o C. Give your answer to two significant figures and with the correct units. [5] ln k = - 9996.2 (1/T) + 19.852 = -13.692 k = e^(-13.692) = 1.13 x 10^(-6) hr -1
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3 Name: _____________________________ 1. (cont.) (f) What is the shelf life of Owlmycin A at 25 o C? Give your answer in weeks to two significant figures. [5] First order, ln ([A]/[A 0 ]) = -kt ln ([0.95 A 0 ]/[A 0 ]) = ln (0.95) = -0.0513 = (- 1.13 x 10^(-6)hr -1 )(t); t = 4.54 x 10^4 hr x ((1 week)/168 hr) t = 270 weeks (g) Will Owlmycin A have a longer or a shorter shelf life at 100 o C compared with that at 25 o C? Explain.
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Exam II - SP09 - Version A - key - 1 Name: _ CHEMISTRY 1032...

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