s11q4Key - 1 Chem 171 Quiz 4 Version A Di-t-butyl...

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Chem 171 Quiz 4 Version A 1. Di-t-butyl peroxide (DTBP) decomposes into acetone (C 3 H 6 O) and ethane(C 2 H 6) by a first order reaction. C 8 H 18 O 2 (g) 2 C 3 H 6 O(g) + C 2 H 6 (g). The reaction is started with pure DTBP at 147 o C and 800.0 mmHg pressure in a flask of constant volume. The half life is 8.0x10 1 min. a) What is the rate constant? b) How much time would it take for the partial pressure of DTBP to be 50.0 mmHg? [A] t = -kt + [A] 0 , ln[A] t = -kt + ln[A] 0 ,1/[A] t = kt +1/[A] 0 , lnk 2 /k 1 = Ea/R(1/T 1 – 1/T 2 ) R=8.314J/mol K, R = 0.0821 Latm/K mol t 1/2 = 0.693/k and k =0.693/ t 1/2 k = 0.693 / 8.0x 10 min = 8.7 x 10 -3 min -1 ln[P] t / P 0 = -kt ln 50/800 = -8.7 x 10 -3 min -1 x t t = 320min Or 4 x t 1/2 = 4 x 80 min = 320 min 2. Butanedione, a main component in the smell and taste of butter and cheese – contains the elements carbon, hydrogen, and oxygen. The empirical formula of butanedione is C 2 H 3 O and its molar mass is 86.09 g/mol. Find its molecular formula. (C =12.01, H= 1.008, O=16.00) Empirical formula mass = 2 (12.01 g/mol) + 3 (1.008 g/mol) + 16.00 g/mol = 43.04 g/mol 86.09/43.04 = 2 Molecular Formula = C 2 H 3 O x 2 = C 4 H 6 O 2 Chem 171 Quiz 4 Version B 1. At what temperature (in K) the t 1/2 for the first order decomposition of N 2 O 5 in CCl 4 is 2.00 h? The rate constant at 25 o C is 3.46 x 10 -5 s -1 . The activation energy is 106 kJ/mol. N 2 O 5 (in CCl 4 ) N 2 O 4 (in CCl 4 ) + ½ O 2 (g) [A] t = -kt + [A] 0 , ln[A] t = -kt + ln[A] 0 ,1/[A] t = kt +1/[A] 0 , lnk 2 /k 1 = Ea/R(1/T 1 – 1/T 2 ) R=8.314J/mol K, R = 0.0821 Latm/K mol
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