Nuclei - Result Question 98

101. A radioactive sample with a half life of 1 month has the label : ‘Activity= 2 micro curies on 1-81991. What would be its activity two months earlier?

[1988]

(a) 1.0 micro curie

(b) 0.5 micro curie

(c) 4 micro curie

(d) 8 micro curie

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Answer:

Correct Answer: 101. (d)

Solution:

  1. (d) In two half lives, the activity becomes one fourth.

Activity on 1-8-91 was 2 micro-curie

$\therefore$ Activity before two months,

$4 \times 2$ micro-curie $=8$ micro curie

Activity of substance decreases as number of undecayed nuclei decreases with time.

Also, Activity $\propto \frac{1}{\text{ Half life }}$