Exponential and Logarithmic Models

Question

The half-life of Palladium-100 is 4 days. After 20 days a sample of Palladium-100 has been reduced to a mass of 4 mg.

What was the initial mass (in mg) of the sample?   

What is the mass 4 weeks after the start?

Answer 1

half life = 4 days

after 20 days 4 mg left, what was the initial amount

P = 128 mg = original amount

20 days = 20/4 = 5 half lives

4x2x2x2x2x2x2 = 4x2^5 = 128 mg originally

day 0 128 mg originally

day 4 128/2 =64

day 8 64/2 = 32

day 12 32/2 = 16

day 16 16/2 = 8

day 20 8/2= 4 mg after 20 days

what is the mass 4 weeks later?? 4 wks = 28 days = 28/4 = 7 half lives

128/2^7 = 1 mg left after 4 weeks