Physics Nuclear Physics NUMS 2020
PMDC Verified Question 50 of 98
Half-life of iodine-131 is 8 days. If 20mg is present initially, how much iodine is left behind after 2 half-lives?
A
10 mg
B
5 mg
C
2.5 mg
D
1.25 mg
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: 5 mg


Concept:

The mass of a radioactive sample halves over each half-life interval, regardless of the actual duration of the half-life.

Solution:

  • Initial mass \(N_0 = 20\text{ mg}\).


  • Number of half-lives \(n = 2\).


  • After 1st half-life (8 days): Mass halves from 20 mg to 10 mg.


  • After 2nd half-life (16 total days): Mass halves again from 10 mg to 5 mg.


Why other options are incorrect:

10 mg is the amount after exactly 1 half-life. 2.5 mg is after 3 half-lives. 1.25 mg is after 4 half-lives.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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