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9 July, 08:45

3.

A rock sample contains 80g of a potassium-40

(K10) isotope with a half-life of 1.25 billion years.

How much of the potassium-40 isotope will remain

after 2.5 billion years have passed?

A. Og

B. 10g

C. 20g

D. 80g

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Answers (1)
  1. 9 July, 12:22
    0
    20 g

    Explanation:

    We are given;

    Original mass of K-40 before decay = 80 g

    Half life of Potassium-40 = 1.25 billion years

    Time taken by the decay = 2.5 billion years

    We are required to calculate the mass of K-10 that will remain after 2.5 billion years.

    Using the formula;

    Remaining mass = Original mass * (1/2) ^n

    where n is the number of half lives

    Number of half lives, n = Time taken : half-life

    = 2.5 billion years : 1.25 billion years

    = 2

    Remaining mass = 80 g * (0.5) ^2

    = 80 g * 0.25

    = 20 g

    Therefore, the mass of K-40 isotope that will remain after 2.5 billion years is 20 g.
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