Does anyone understand half-lifes? *physicaly science*?

Question

here are the questions, you might have to describe to me how to do it though pleaseeeeeeeeeeeeeeeeeee <33
1. After about how many half-lives would a small sample of radon *life is 3.82 days* be "gone?" Why?
2.If your basement has radon, the radon still wont be gone after dozens of half lives. Why not?
3. 250 g of I-131 emits beta particles in its radioactive decay. it's half life is 8 days. how much remains after 16 days? wrtie the decay equation.
4. the half life of Sr-90 is 28 years. if 60 g of Sr-90 is found in a soil sample, estimate the amount in the sample after 84 years. write as decay equation (emits beta particle)
5. Sr-82 has a half life of 25 days. if you begin with a 140 g sample, how many days later will you find 17.5 g?
6. the half life of a certain isotope is 20 h. how much of an original sample of 320 g remains after 80 h? after 5 d?
7. N-13 has a half life or 10 minutes. if you start with 2 grams, how much remains after 40 mins?
Help me please!!!!!!!!! :] explain plz!

Answer 1

The first poster has the right definition. As for your questions:

1) Mathematically, the substance will never be "gone". You will always have half of what you had one half-life ago. There are thumbrules for this - generally, if you wait five half-lives, you consider that the sample has decayed completely. However, you might want to check your class notes as your teacher may have decided to use a different standard.

2) See the discussion in (1) above. It will never be completely gone in a mathematical sense. Physically is a different story, but you probably don't want to get into that here.

3) After sixteen days, you've gone through two half-lives. That means that you have (1/2)(1/2) = 1/4 of what you started with. One quarter of 250 g is 62.5 g. There are several different possible forms for the decay equation, depending on what you're using as a base - check your textbook and your class notes.

4) The half-life is 28 years and you're looking at it after 84 years. This is 84/28 = 3 half-lives. That would mean you're looking at (1/2)(1/2)(1/2) = 1/8 of the original sample. Since the sample is now 60 g, originally you had 8 * 60 g = 480 g.

I'm going to let you work through the rest of them. You'll need to get familiar with the specific form of the decay equation that you're using in class to answer the rest of them.

Answer 2

1. undetermined, half life is the time for half of a large sample to degrade, on a small sample (2 atoms) there is no way of telling when either will degrade.

2. .5X.5X.5X.5X.5= measuarble amount especially if imagined how many atoms in a gram, .5^25=0.0000000298023223876953125

3. 67.5 g ( 250x0.5^(16/8) ) don't know decay equation

4. 60x(.5^(84/28))=
60x0.5^3=
60x.125=7.5 g easy

5. 17.5/140=.125=.5^3
time is 3x25=75 days easy

6. 320x(.5^(80/20)=
320x(.5^4)=
320x.0625=20 easy

320x(.5^(120/20)=
320x(.5^6)=
320x.015625=5 easy

7. follow above examples and good luck!

Answer 3

Half life simply means that during that period, one-half of the substance in question is gone. for example, a 10 gm substance with a half life of 10 days, will be down to 5 gms in 10 days, 2.5 gms in 20 days, 1.25 gms in 30 days. It reduces in half every 10 days.