1.How many nuclei of 238 92 U remain in arock if the activity registers 250 deca

Question

1.How many nuclei of 23892U remain in arock if the activity registers 250 decays per second? 2.2455Cshas a half-life of 30.8 s. (a) If we have 8.8 µginitially, how many Cs nuclei are present?

(b) How many are present 2.0 min later?

(c) What is the activity at this time?
decays/s
(d) After how much time will the activity drop to less than about 1per second?
min
2.2455Cshas a half-life of 30.8 s. (a) If we have 8.8 µginitially, how many Cs nuclei are present?

(b) How many are present 2.0 min later?

(c) What is the activity at this time?
decays/s
(d) After how much time will the activity drop to less than about 1per second?
min

Explanation / Answer

   the decay constant is given by
   = 0.693 / T1/2
      = 0.693 / 30.8 s
      = ........ s-1
(a)
   the initial number of nuclei will be
   No = (8.8 x 10-6 g) (6.02 x1023 atoms / mol) / (124 g / mol)
         = ........nuclei
(b)
   when t = 2.0 min the exponent will be
   t = () (2.0 min) (60 s / min)
        = .......
   N = No e-t
       =  ........nuclei   
(c)
   the activity will be
   N = ..........
(d)
   the time is given by   
   N = Noe-t
   t = ........ min

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