1.

The half-life of the isotope `._11 Na^24` is `15` hrs. How much time does it take for `(7)/(8) th` of a sample of this isotope to decay ?A. 75 hrsB. 65 hrsC. 55 hrsD. 45 hrs

Answer» Correct Answer - D
(d) Undecayed isotope `= 1 - (7)/(8) = (1)/(8)`
`:. (N)/(N_0) = ((1)/(2))^(t//T) rArr ((1)/(8)) = ((1)/(2))^(t//15) rArr (t)/(15) = 3`
or `t = 45 hours`.


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