i so need help on half life problems?

Question

i need help on these 2 problems!
1. If a 2.0 grams radioactive substance has a half-life of 20 minutes, how many half-lives have occurred in 120 minutes?

2. If a 2.0 grams radioactive substance has a half-life of 20
minutes, how much remains after 120 minutes?

Answer 1

1) simple arithmatical problem, 120/20=6 half lives have occured.

2)the formula is , the amount of substance left = a/2^n
a= initial wt of substance
n= no of half lives.
so here,the amount of substance left=2/2^6 = 1/32=0.03125 gm.

Answer 2

1. There is a half life every 20mins in 120mins there will be 120/20 = 6
2. original 2g
1st halflife 1g, 2nd 0.5g, 3rd 0.25g, 4th 0.125g, 5th 0.062775g, after the 6th (last) halflife there will be 0.0313875g of radioactive substance left.

All you have to do is remember after 20mins the amount will half, and will continue to do this until it has gone.