Isotopes Frequently Used in Radiometric Dating Radioactive Parent Stable Daughte

Question

Isotopes Frequently Used in Radiometric Dating Radioactive Parent Stable Daughter Half Life Values Product U-238 U-235 Th-232 Rb-87 -40 PD-206 PD-207 PD-208 St-87 Ar-40 -14 4.5 Billion Years 713 Million Years 14.1 Billion Years 7 Billion Years 1.3 Billion Years 14 730 Years 7. A sample of organic material from an archeological site in Egypt contains 125 atoms of C-14 and 375 atoms ofN-14. How many half-lives have passed? What fraction of the original C-14 in a sample is left after 11,460 years? A very large sample originally contained 8 g of potassium-40, but now contains only 0.25 g of that 8. 9. isotope. How many half-lives have passed? 10, How old is a sample that contains 25% of its original K40? 11. Nickel-63 has a half-life of 92 years. At the end of 368 years, how much nickel-63 would remain in a sample initially containing 20 micrograms? 12. A rock contains 4 mg of a radioactive isotope. At the end of 60 years, only 0.5 mg of the element remains in the rock. What is the half-life of the element? 13. Calcium-41 has a half-life 80,000 years. At the end of 320,000 years, how much calcium-41 would remain in a sample initially containing 240 micrograms?

Explanation / Answer

7.)Current sample contains 125 C-14 atoms and 375 N-14 atoms.

before one half life there would have 250 C-14 atoms and 250 N-14 atoms.

again before 2 half lifes there would have 500 C-14 atoms and no N-14 atoms.

So, we have 2 half lives.

8.) Since there are 2 half lifes involved ( 11460 = 5730 x2)

So, after first haf life it would be 50% C -14 left and again after another 25% would remaining.

fraction left = 0.25

9.) 8 = (0.25) x 2t

or, 2t = 32

or, t = 5

so, no of half lives involved are 5.

10.) 100 = 25 x (2)t

or, 4 = 2t

or, t = 2

so there are 2 half lives

therefore time involved = 2 x 1.3 billion years = 2.6 billion years

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