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What fraction of an original sample of 131I remains unchanged after 24.063 days?

June 2023 NY Regents Chemistry Exam

48 What fraction of an original sample of 131I remains unchanged after 24.063 days? (1) 1/8

(2) 1/2

(3) 1/3

(4) 1/4


Solution: Table N on the Reference Table shows half lives for different radioisotopes. The half life for I-131 is 8.021 days. 24.063/ 8.021 = 3 half lives.

How much of the sample will be left after three half lives? After each half live the amount of substance decreases by 1/2.

1 --> 1/2 --> 1/4 -->1/8

Answer: 1


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