12. A rock contains 4mg of a radioactive isotope. At the end of 60 years, only 0

Question

12. A rock contains 4mg of a radioactive isotope. At the end of 60 years, only 0.5 mg of the elements remain in the rock. What is the half life of the element? 13. Calcium-41 has a half life of 80,000 years. At the end of 320,000 years how much calcium-41 would remain in a sample initially containing 240 micrograms? 10. How old is a sample that contains 25% of its original K-40? 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 adioactive 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

10. half life of K 40 is 1.3 billion years

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

11. in 368 years there are 4 half llives

so, 20 = x (2)4

or, x = 20/16 = 1.25 micrograms

12. 4= 0.5 * 2t

or, 8 = 2t

or , t = 3

So , in 60 years there are 3 half lives

so one half life = 60/3 years = 20 years

13. no. of half lives = 320000/80000 = 4

240 = x (2)4

or, x = 15

So, remaining Ca 41 will be 15 micrograms

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