A radioactive sample has 72 g of a nuclide. How much remains after 3 half-lives?

Answer and explanation

Correct answer: D. 9 g

  1. Step 1: After each half-life the mass is halved, so the remaining mass is N0 / 2^n.
  2. Step 2: 72 / 2^3 = 9 g.
Answer
9 g.
Why not the others: '24 g' divides by the number of half-lives instead of 2^n; '12 g' divides by 2n instead of 2^n; '18 g' counts one half-life too few.

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