The amount of radioactive substance left after 75 hrs, when 100g of it is taken initially, will be: (half life of radioactive substance is 15 hrs)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 3.125 g
All radioactive decay follows first order kinetics, so the half-life is constant and does not depend on how much substance is present. After every half-life the amount is halved; after n half-lives:
\( \displaystyle N = \frac{N_0}{2^{n}} \)
Number of half-lives in 75 h:
n = 75 h / 15 h = 5
Amount left:
N = 100 g / 25 = 100 g / 32 = 3.125 g
(100 → 50 → 25 → 12.5 → 6.25 → 3.125 g)