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2015-04-03T10:11:09+05:30

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Half life = 19.7 minutes

 t_{\frac{1}{12}}=\frac{ln\ 2}{\lambda}\\ \\ \Rightarrow 19.7 = \frac{ln\ 2}{\lambda}\\ \\ \Rightarrow \lambda = \frac{ln\ 2}{19.7}\\ \\ \Rightarrow \lambda = \frac{0.693}{19.7}\\ \\ \Rightarrow \lambda = 0.0351\ Bq

So decay constant is 0.0351 Bq.

half life = 19.7 minutes
number of half lives in 78.4 minutes = 78.4/19.7 = 4
initial amount (N₀)= 1g
let after 4 half lives, remaining will be = N_4

N_4= \frac{N_0}{2^4} = \frac{1}{16}\ g

So after 78.4 minutes,  \frac{1}{16}  g 214Bi will be remaining.
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