Radioactivity
Review
Types of Decay?
Half-Life
What does this mean?
Create your own definition from Mine.
Practice example
Mercury-195 has a half life of 31 hours.
If you start with 5.00g of pure mercury195, how much of it will still be present
after 93 hours?
What do you Know?
Mercury-195
195
90Hg
half life = 31 hours
you have 5.00g of Hg to start with
93 hours pass
What are we looking for?
How much do we have left after 93
hours?
Solve:
93/31= 3 half lives
(time pass/half life= # of half lives)
We know that the initial mass will be
divided in half 3 times.
first time)
5.00g/2= 2.50g
Second Time)
2.50g/2= 1.25g
Third Time)
1.25/2= 0.625g
0.625g will be left after 93 hours.
Practice on your
own:
Problem 1:
Gold-191 has a half -life of 12.4 hours.
What mass of this isotope would remain
after 49.6 hours if you started with
7.50mg sample of pure Au?
Answer problem 1
49.6/12.4 = 4 half lives
1) 7.50/2 = 3.75mg
2) 3.75/2 = 1.875 mg
3) 1.875/ 2= 0.9375mg
4) 0.9375/2 = 0.46875 mg
after 49.6 hr there will be 0.46875 mg
left of the original sample.
Problem 2
Iodine-129 with an original mass of
15.000mg loses 13.125 mg of the
original mass in 39 days. What is the
half life?
Answer to problem
2
Original mass = 15.000
Finial mass = 15.000 - 13.125= 1.875
mg
39 days pass
1 half life = 15/2 = 7.5
2 half life = 7.5/2 =3.75
3 half life = 3.75/2= 1.875
39/3 =13days