31. is a B emitter with a half-life of 20 years. How much of a 250 g sample of c

Question

31. is a B emitter with a half-life of 20 years. How much of a 250 g sample of cesium-134 will remain after 10 years? A) 0.0024 g B) 0.078 g C) 0.25 g D) 050 g E) none of the above 32. An ele process has an energy of 88 imol.1f the enthalpy change for the reaction is -40 kl/mol. what is the activation energy for the reverse reaction? d) -88 kJlmol e) 128 kJ/mol 33. If a reaction were endothermic, would the forward or the reverse reaction have a higher activation energy? A) One would need absolute rate data determine this. B) The forward and reverse activation energies are equal. C) The forward activation energy D) The reverse activation energy E) none of these 34. What is the time needed for the activity of a gold 198 source to change from 25 ci to 20ci The half-ife of gold-198 is 64.6 h. a) 301 h b) 236 h c) 0.0391 h d) 808 h e) 103 h

Explanation / Answer

31) Half life = 2 years.

Initial concentration = 2.50 g.

The total time = 10 years

numbers of half lifes (t1/2) = 10/2

= 5.

2.50 g...1..........1.25 g......2.......0.625 g......3.......0.3125g......4...........0.1562g.....5...........0.0781g

Ans:(b)

32) Ans:(d)

33) Ans: (b)

34) K = 2.303/t log a/(a-x)

k=0.693/t1/2   t1/2=64.6 h

K= 0.693/64.6

K = 0.0107 sec-1.

t= 2.303/K log a/(a-x) a= 25 (initial concentration).

a-x = 2 (remaining concentration).

t=2.303/0.0107 log 25/2

t = 215.23 x 1.0969

t = 236 h

Ans: (b)

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