Uranium-238 forms thorium-234 after radioactive decay and has a half-life of 4.5 x 10^9 years. How many years will it take to decay 75% of the initial amount?
Correct answer: A. 9 x 10^9 years
- A. 9 x 10^9 years
- B. 4 x 10^9 years
- C. 9 x 10^10 years
- D. 4 x 10^10 years
Explanation
Half Life of radioactive element is defined as the time taken by the sample to decay 50% of its initial value. As asked in question for time taken to decay 75% of the initial amount, so For 1st Half Life : Uranium-238 decays 50% and 50% remains undecayed. For 2nd Half Life: Rest of 50% of undecayed Uranium-238 decays to half i.e 25% decays and 25% remains undecayed In two Half Lifes total of (50% + 25%) 75% Uranium decays. Total time taken to decay 75% of the initial amount = 2 Half Life = 2 x (4.5x109) = 9 x 109 years
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