OpenStudy (anonymous):

A substance has a half-life of 60,000 years and an initial mass of 1,000 grams. How many years will pass before only 250 grams of the substance's parent material is left? 30,000 90,000 120,000 240,000

OpenStudy (kainui):

First off, what is a half life? It's the amount of time it takes for an initial amount to decay to half an amount. So if fairy dust has a half life of 10 years and you have 8 pounds of it, you'll only have 4 pounds of it in 10 years. Another 10 years and you'll have only 2. Another 10 pounds you have 1. Get it? So do something similar here.

OpenStudy (anonymous):

no :l

OpenStudy (kainui):

yes :|

OpenStudy (kainui):

What don't you get? You have to think about it, and I can't think for you I'm afraid.

OpenStudy (anonymous):

I think its 30 000

OpenStudy (kainui):

Let's go back to the definition of half-life. What is it?

OpenStudy (anonymous):

decay the half amount

OpenStudy (mertsj):

Are you expected to use a formula?

OpenStudy (kainui):

I doubt it since 250 is exactly two half lives away from 1000.

OpenStudy (anonymous):

Nope

OpenStudy (mertsj):

To whom are you saying "Nope"?

OpenStudy (kainui):

He's not using complete sentences or even trying. He's on his own now.

OpenStudy (anonymous):

To Kainui

OpenStudy (mertsj):

Did you do step 1 and define the variables?

OpenStudy (anonymous):

No, as I am lost.

OpenStudy (mertsj):

What is the beginning amount?

OpenStudy (mertsj):

A substance has a half-life of 60,000 years and an initial mass of 1,000 grams.

OpenStudy (anonymous):

120 000 ?

OpenStudy (mertsj):

Do you know what "initial" means in the sentence above where it says "an initial mass of 1000 g"?

OpenStudy (anonymous):

Oh i meant years, so its 1000 g?

OpenStudy (mertsj):

Yes. the beginning amount is 1000 g Now what is the elapsed time?

OpenStudy (anonymous):

60000

OpenStudy (mertsj):

How many years will pass

OpenStudy (mertsj):

That is the question the problem asks. The elapsed time is what you are supposed to find.

OpenStudy (mertsj):

What is the half life?

OpenStudy (anonymous):

half life of 60 000 would be 30 000, or did you mean half life as 60 000

OpenStudy (mertsj):

"A substance has a half-life of 60,000 years." That's what the problem says. So I ask you...what is the half-life?

OpenStudy (anonymous):

60 000

OpenStudy (mertsj):

What is the ending amount?

OpenStudy (anonymous):

Ending amount..?

OpenStudy (mertsj):

"only 250 grams of the substance's parent material is left." If only 250 grams is left, take a wild guess...what is the ending amount?

OpenStudy (anonymous):

ending amount would be 0 as far as im concerned :/

OpenStudy (anonymous):

Oh wait is it 250 *

OpenStudy (mertsj):

Let's say you have $100 and you go Christmas shopping and when you are done, you have only $10 left. What is you "ending amount" of money?

OpenStudy (anonymous):

10

OpenStudy (mertsj):

Now let's make sure you understand half life. Let's say you have $1000 and every 4 days you spend half of your money. That means that the half life of your money is 4 days. It is now Sunday night. How much money will you have left Thursday night?

OpenStudy (anonymous):

500

OpenStudy (mertsj):

And what about the following Monday night?

OpenStudy (anonymous):

250

OpenStudy (mertsj):

And the following Friday night?

OpenStudy (anonymous):

62.5

OpenStudy (mertsj):

What planet are you living on where half of $250 is $62.50?

OpenStudy (anonymous):

125 my bad, was thinking about the 4 days

OpenStudy (mertsj):

Now. Let's go back to your problem. You start with 1000 g of this stuff. The half life is 60,000 years. How much stuff will be left after 60,000 years?

OpenStudy (anonymous):

So the start was 120 000 ?

OpenStudy (mertsj):

The start of what?

OpenStudy (anonymous):

Half life of 60k so 120/2 = 60

OpenStudy (mertsj):

So here's your trouble. You don't know what half life means.

OpenStudy (mertsj):

Half life is a period of time.

OpenStudy (mertsj):

It's the amount of time it takes for half of something to disappear. For example. When I bake a pie, it's half life is about 30 minutes.

OpenStudy (mertsj):

That means my family eats half of it in 30 minutes.

OpenStudy (anonymous):

k

OpenStudy (mertsj):

So in your problem, because this stuff is radioactive and is decaying, it takes 60,000 years for half of it to disappear.

OpenStudy (mertsj):

So we start at time 0 with 1000 grams. How many grams will be left after 60,000 years?

OpenStudy (mertsj):

|dw:1387772064668:dw|