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Question

If 78th of the initial mass of a radioactive substance decays in 15 days, then what is the half-life period of the radioactive substance?

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Solution

Let the initial number of radioactive nuclei be N0.
Then, after a half-life period, the number of nuclei present would be N0×12.
After two half-lives, the number of nuclei present would be 12×(N0×(12))=N0(12)2
Similarly, after n half-lives, the number of nuclei present would be N=N0(12)n (1)
If t is the given time period and T is the half-life period of the radioactive substance,
then n=tT (2)
Also, if m0 is the initial mass and m the mass of undecayed nuclei of the substance,
then NN0=mm0 (3)
Equation (1) can be written as
m=m0(12)n or m=m0(12)tT (4)
In the given problem, undecayed mass
m=m078m0=18m0 ; t=15 days, T= ?
Substituting the values of m,m0 and t in equation (4), we get
18m0=m0(12)15T18=(12)15T(12)3=(12)15T
15T=3T=153 days =5 days

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