It took 109.8 years for a 300.0 mg sample of an unknown radioactive material to completely disintegrate. Calculate the mass of the element (in grams/mole) assuming that the disintegrations per second are constant over the life-time of the sample, and the sample is labeled 100.0 Ci (Curie= 3.7x10^10 disintegrations per second)

This material has a half-life. It NEVER completely disintergrates.

That is what is written in the problem.

The assumptions here are wild.

OK. If all of the atoms are going to "disintegrate", and they do so at a "constant rate" independent of mass, then

Time= numberatoms/rate

The number of atoms= avagNumber*moles=

and then you solve for molmass.

time= avagnumber*mass/molmass*rate

You are given time, avagnumber, mass, and rate. Make certain you have time and rate in consistent units.

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