Need help now Approximate half-life
Radon-222 = 4 days

A sample contains 18mg of radon-222. You estimate that this is 0.01% of the original isotope. How many half-lives have passed?

13 half-lives have passed

General formulas used in decay:  

T = duration of decay  

t 1/2 = half-life  

N₀ = the number of initial radioactive atoms  

Nt = the number of radioactive atoms left after decaying during T time  

t1/2 = 4 days

Nt=18 mg (0.01% of the original isotope)

18 mg (Nt) = 0.01% No

No = the original isotope :

The duration of decay  (T) :

Half-lives passed :

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)