Page 497 - Chemistry eBook

Base your answers to questions 82 through 85 on the information below and on your knowledge of chemistry.
Iodine has many isotopes, but only iodine-127 is stable and is found in nature. One radioactive iodine isotope, I-108, decays by alpha particle emission. Iodine-131 is also radioactive and has many important medical uses.
82 Determine the number of neutrons in an atom of I-127. [1]
Sample Regents Questions
83 Explain, in terms of protons and neutrons, why I-127 and I-131 are different isotopes of iodine. [1]
84 Complete the equation in your answer booklet for the nuclear decay of I-108. [1]
85 Determine the total time required for an 80.0-gram sample of I-131 to decay until only
1.25 grams of the sample remains unchanged. [1]
Jan 2014 #85
Answer
P.S./Chem.–Jan. ’14 [13] 48.13 days
Explanation:
1. Let’s figure out how many half-lives happened for an 80-gram sample to change into a 1.25-gram sample:
2. It takes 6 half-lives. How long are 6 half-lives for I-131? Table N (p. 6) shows that one half-life for I-131 is 8.021 days. Six half-lives = 6 X 8.021 = 48.126 days
1 half-life
2 half-lives
3 half-lives
4 half-lives
5 half-lives
6 half-lives
80 = 40 2
40 = 20 2
20 = 10 2
120 = 5
25 = 2.5
2.5 = 1.25 2
UNIT 12: NUCLEAR CHEMISTRY © 496