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?
(1) 1.04 s (3) 4.16 s
(2) 2.08 s (4) 8.32 s

4.16s

Explanation:

N = 20*10⁻³g

N₀ = 80*10⁻³g

t = 8.32

N = N₀e⁻λt

In(N/N₀) = -λt

-λ = 1/t * In(N/N₀)

-λ = 1 / 8.32 * In (20*10⁻³ / 80*10⁻³)

-λ = 0.12 * In(0.25)

-λ = -0.167

λ = 0.167

t½ = 0.693 / λ

t½ = 0.693 / 0.167

t½ = 4.16s

c. 75.7

i think that is the right