An original sample of the radioisotope fluorine-21 had a mass of 80.0 milligrams. Only 20.0 milligrams of this original sample remain unchanged after 8.32 seconds. What is the half life of fluorine-21?
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You are starting with 80.0 mg of F-21.
After 1 half-life, you will have 40.0 mg of F-21.
After 2 half-lives, you will have 20.0 mg of F-21.
Hence, 2 half-lives=8.32 s.
$\therefore$ 1 half-life = 4.16 s.