An unknown radioactive substance has a half-life of 3.20 hours. If 16.5 g of the substance is currently present, what mass A0 was present 8.00 hours ago

the amount present 8 hours ago is Ao = 93.338 g

Explanation:

the radioactive decay is given by

A = Ao * 2^ ( - t/T)

where T = half-life of the substance, t = time, A = amount of substance at time t, Ao = amount of substance at time 0

therefore at t=8 hours, A=16.5 gr

Ao = A*2^ (t/T) = 16.5 g * 2^ (8 hours/3.20 hours) = 93.338 g

Ao = 93.338 g