SAT Subject Physics Practice Question 212: Answer and Explanation

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Question: 212

58. Lead-199 has a half-life of 1.5 hours. If a researcher begins with 2 grams of lead-199, how much will remain after 6 hours?

A. 0.125 grams
B. 0.25 grams
C. 0.375 grams
D. 0.5 grams
E. 0.625 grams

Correct Answer: A


Because each half-life is 1.5 hours, a time interval of 6 hours is equal to 4 half-lives. After each half-life elapses, the mass of the sample is cut in half, so after 4 half-lives, the mass of the sample decreases from 2 grams to 1 gram to 0.5 grams to 0.25 grams and, finally, to 0.125 grams.

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