Radiometric Dating

[匿名]

2009-10-04 12:31 pm

According to
Dating System Age of half-life
238 U>206Pb 4.5 billion years
235U>207Pb 713 million years
40K>40Ar 1.3 billion years
14C>14N 5730 years

1. A rock originally had 1000 atoms of 238U and it now has 500.
How old is it? (please show the work)

2. A rock originally had 2400 atoms o 40K and no 40Ar.It now has
200 atoms of 40K. Assumig that the only chemical process
is radioactive decay of potassium to argon, what is the age of the rock?

a) 1.57 billion years
b)1.375 billion years
c) 2.643 billion years
d) 5.303 billion years
e) 4.660 billion years

How many atoms of 40Ar are there in the rock now?

3. A rock originally has 1200 atoms of 235U and no 207Pb. It now
has 220 atoms of 235U. Assuming the only chemical process is the
radioactve decay of uranium to lead, what is the age of the rock?

a) 1.745 billion years
b) 1.375 billion years
c) 2.643 billion years
d) 1.032 billion years
e) 713 million years

How many atoms of 207Pb are there in the rock now?

please show me the works of these problems!! I reallly don't know how to do><
thanks a lot!!!

回答 (1)

Prof. Physics

2009-10-04 7:09 pm

✔ 最佳答案

1. Half of the U-238 atoms remain, so it has passed the half-life of U-238.

So, it is 4.5 billion years old.

2. Decay constant, k = ln2/t1/2 = ln2 / 1.3 /billion years

By N = N0e-kt

200 = 2400e-(ln2 / 1.3)t

Solving, Time, t = 4.660 billion years

The answer is E.

3. Decay constant, k = ln2 / t1/2 = ln2 / 713 / million years

By N = N0e-kt

220 = 1200e-(ln2 / 713)t

Solving, time, t = 1745 million years = 1.745 billion years

The answer is A.

The number of atoms should conserve.

So, the number of 207Pb atoms = 1200 - 220 = 980

參考: Physics king

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