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A sample initially contains 8.0 moles of a radioactive isotope. How much of the sample remains after four half-lives? Express your answer using two significant figures.

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Answer:

0.50 mol

Explanation:

The half-life is the time required for the amount of a radioactive isotope to decay to half that amount.

Initially, there are 8.0 moles.

  • After 1 half-life, there remain 1/2 × 8.0 mol = 4.0 mol.
  • After 2 half-lives, there remain 1/2 × 4.0 mol = 2.0 mol.
  • After 3 half-lives, there remain 1/2 × 2.0 mol = 1.0 mol.
  • After 4 half-lives, there remain 1/2 × 1.0 mol = 0.50 mol.

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