NEET Physics Nuclei Questions Solved

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NEET - 2014

A radio isotope X with a half life 1.4x109 yr decays of Y which is stable. A sample of the rock from a cave was found to contain X and Y in the ratio 1:7. The age of the rock is

(a)1.96x109 yr

(b)3.92x109 yr

(c)4.20x109 yr

(d)8.40x10yr

Ratio of X:Y is given=1:7 

mx/my=1/7

=> 7mx/my

=> Let the initial total mass is m.

=> mx+my=m => my/7+my=m

=> 8my/7=m

=> my=7/8m

only 1/8 part remains 

=> 1->1/2->1/4->1/8
           T/2    T/2   T/2

So,time taken to become 1/8 unstable part

=3 x T1/2 = 3 x 1.4 x 109 =4.2 x 109 y


Difficulty Level:

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