The radioisotope radon-222 has half-life of 3.8 days. how much of a 73.9-g sample of radon-222 would be left after approximately 23 days?

This can be solved by the equation: 

A = Aoe^(-ln2/t)T
where Ao is the initial amount of the substance ( 73.9-g sample of radon-222 ), t is the half-life time (3.8 days) and T is the time elapsed. Plugging in the values in the equation, the substance left is 1.11g. 

The correct name for n3-ion should be azide.