Question

3. Uranium-238 decays into lead. For a given amount of 238U, half is converted to lead every 4.5 million years.

a) If a sample is found with 1/8 (0.125) of the expected 238U, how old would this sample be? (2 points)

b) For carbon-14, 60,000 years is the maximum age that can be accurately dated. What would be the maximum age of a sample if 238U were used? (3 points)

Answer 1

In 4.5 million years, the amount drops to 1/2

In another 4.5 million years, it'll be 1/2*1/2...

Answer 2

Keep going until it becomes 1/8th

Answer 3

So just divide by 4.5 million years

Answer 4

Not at all.

In 4.5 million years, the amount drops to 1/2

In another 4.5 million years, it'll be 1/2*1/2

In another 4.5 million years...

Answer 5

1/2 *1/2*1/2

Answer 6

Yep, now look back at a), and answer it.

Answer 7

Well I need to get to 1/8 right

Answer 8

1/2 *1/2*1/2 = ?

Answer 9

0.125

Answer 10

Right but that isn't the answer. How much TIME did it take? Look back through the posts above.

Answer 11

13.5 million years

Answer 12

For b) you need to look back at one of the other carbon 14 questions you had

Answer 13

"For carbon-14, 60,000 years is the maximum age" how many half lives was that?

Answer 14

6.000

Answer 15

No, how many half lives was there in 60,000 years?

Answer 16

So the age would just be 6,000

Answer 17

You already worked this out once before. If the half life is 6000, then how many half lives is there in 60K years.

Answer 18

10

Answer 19

So it would just be 10years

Answer 20

Nope... think about what the question is asking.

If the half life is 6000, then how many ***half lives*** is there in 60K years?

Answer 21

Um wouldn't it be 60000/6000

Answer 22

Yes. But the answer is not 10 YEARS. Read the question carefully.

Answer 23

oh half lives

Answer 24

So 10 half lives

Answer 25

Yes. Now use that
For carbon-14, 60,000 years is the maximum age that can be accurately dated. What would be the maximum age of a sample if 238U were used?

Answer 26

Um well we know that 238U, half is converted to lead every 4.5 million years.

Answer 27

"For carbon-14, 60,000 years is the maximum age that can be accurately dated" - how many half lives is that?

"What would be the maximum age of a sample if 238U were used?" - same number of half lives, how much time would that be?

Answer 28

divide 238/10 right

Answer 29

No, what is the half life of U238?

Answer 30

119

Answer 31

It tells you in the question. U238 is the name of the element, nothing to do with half life.

Answer 32

"For carbon-14, 60,000 years is the maximum age that can be accurately dated." - we know this is 10 half lives

"What would be the maximum age of a sample if 238U were used?" - they are asking how long 10 half lives of U238 is

Answer 33

10*268 then,

Answer 34

238

Answer 35

"U238 is the name of the element, nothing to do with half life."

Answer 36

Half life of U238 is given in the question

Answer 37

lead every 4.5 million years.

Answer 38

So the half life is?

Answer 39

"For carbon-14, 60,000 years is the maximum age that can be accurately dated" - how many half lives is that?

"What would be the maximum age of a sample if 238U were used?" - same number of half lives, how much time would that be?

Answer 40

@mathmate

Answer 41

I just don't understand what he means

Answer 42

What the question is telling us is that we can date using carbon-14 for up to 10 half-lives, right?

Answer 43

Yes

Answer 44

ten half-lives of carbon-14 equal 10*6000=60000 year, right?

Answer 45

Right

Answer 46

Since the half-life of C-14 is only 6000 years, we could date a longer period if the half-life had been long, do you agree?

Answer 47

*longer

Answer 48

yes, I do agree

Answer 49

Well, I have to go, so I will finish my reasoning for you to think about.
The question seems to say that 10 half-lives is what's limiting the dating process. So if the half-life had been longer, say, a million years, we would have been able to date up to 10 million year.

So part b of the question is asking if U238 was used, which has a half-life of 14.5 million years, how far back can we date objects?

Answer 50

Well to see how far back you would divide right

Answer 51

half-life of 6000 years, you can date up to 10 half-lives, so 10*6000 years=60000 years.
Now decide on the half-life of U238, and then we can date up to 10 half-lives of U238, and that equals how many years?

Answer 52

1428000

Answer 53

238*6000

Answer 54

@phi

Answer 55

which part are you doing ?

Answer 56

b

Answer 57

Do you know the half-life of C-14 ? (It does not tell me in the question)

Answer 58

6000

Answer 59

60,000 years is the maximum age (using C-14)

They are saying that after 60,000 years, the Carbon has disintegrated to almost nothing (too small to measure accurately).

How many half-lives is 60,0000 ?
in other words, how many times does 6000 go into 60,000 ?

Answer 60

Math mate found the answer up above to

How many half-lives is 60,0000 ?
in other words, how many times does 6000 go into 60,000 ?

Answer 61

10

Answer 62

So it sounds like the longest we can go is 10 half-lives
each radioactive element has a different half-life, but what ever it is, we can only measure back 10 of them.

Answer 63

So the answer is ten?

Answer 64

I think they want the answer in years. The answer is 10 half-lives (for U238)
what is that in years ? any idea ?

Answer 65

60,000 years

Answer 66

I guess you don't have the idea.
what is the half-life of U238 ? (they tell you in the problem)

Answer 67

It just says For a given amount of 238U, half is converted to lead every 4.5 million years.

Answer 68

ok, and that means half of the 238U disintegrated (into lead) after 4.5 million years
that is telling you the "half-life" (how long it takes for ½ of the stuff to disappear)

Answer 69

yes

Answer 70

what is the half-life of U238 ?

Answer 71

4.5?

Answer 72

ugh I'm so confused :/

Answer 73

you are doing good.
but not just 4.5 years

Answer 74

4.5 million years

Answer 75

yes. the half-life of 238U is 4.5 million years.

now we need to know how many years is 10 half-lives ?

Answer 76

oh so 10*4.5

Answer 77

yes, if you add million years at the end

Answer 78

45million years

Answer 79

yes, you got it.

Answer 80

Thank you so so so much, I really do appreciate it

Answer 81

yw