radioactive nuclei of an element decay at a characteristic, constant rate
the time it takes for half of radioactive sample to decay
number of radioactive nuclei remaining is halved after each half life
different isotopes have different half lives
Slide 2
Half-life Equation
Mf=mi(1/2)nn=ttot t1/2
Mf= final massmi= initial massn= number of half lives
Slide 3
Half-life Example 1
How much of a 100.0 g sample of Au is left after 8.10 days if its half-life is 2.70 days?answer: 12.5g
mf= ?mi= 100.ogttot= 8.10 days t1/2= 2.70 daysn= 3 days n=ttot 8.10 days =3 days t1/2 2.70 days mf=100.0g(1/2)3 100.0g x 0.125 =12.5g
Slide 4
Example 2
A 50.0 g sampler of N decays to 12.5 g in 14.4 seconds. What is its half life?answer: 7.2g
mi=50.0gmf=12.5gttot= 14.4 sect1/2=?n=2 50.0 g 25.0g 1 12.5g 2 n=ttot t1/2=ttot 14.4 sec t1/2= 7.2g t1/2 n 2
Slide 5
Example 3
There are 5.0 g of I left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days?answer: 160 g
mf= 5.0 gmi= ?t1/2= 8.07 daysttot= 40.35 daysn= 5 dyas n=40.35 days n=5 days mf=mi(1/2)n 8.07 days mi=mf 5.0g 5.0g (1/2)n (1/2)5 0.03125 mi= 160g
