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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Answer 1 · 2016-03-06T00:37:59

The half-life of fluorine-21 is 4.16 s.

You are starting with 80.0 mg of $F-21$ $"F-21"$ .

After 1 half-life, you will have 40.0 mg of $F-21$ $"F-21"$ .

After 2 half-lives, you will have 20.0 mg of $F-21$ $"F-21"$ .

Hence, $2 half-lives = 8.32 s$ $"2 half-lives = 8.32 s"$ .

∴ $1 half-life = 4.16 s$ $"1 half-life = 4.16 s"$ .