A geologist in Guyana is dating a rock sample using radiometric methods. The sample originally contained 80 g of a radioactive isotope. After 3 half-lives, what mass of the original isotope remains?

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Question

A geologist in Guyana is dating a rock sample using radiometric methods. The sample originally contained 80 g of a radioactive isotope. After 3 half-lives, what mass of the original isotope remains?

Accepted Answer

10 g — Award 1 mark for correct calculation. After 1 half-life: 80 g ÷ 2 = 40 g. After 2 half-lives: 40 g ÷ 2 = 20 g. After 3 half-lives: 20 g ÷ 2 = 10 g. A is incorrect — this is after 1 half-life. B is incorrect — this is after 2 half-lives. D is incorrect — this would be after 4 half-lives.

Answer

40 g

Answer

10 g

Answer

20 g

Answer

5 g

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