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22 August, 09:38

A radioisotope of technetium is useful in medical imaging techniques. A sample initially contains 80.0 mg of this isotope. After 24.0 h, only 5.0 mg of the technetium isotope remains. What is the half-life of the isotope?

a. 3.0 h

b. 6.0 h

c. 12.0 h

d. 16.0 h

e. 24.0 h

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Answers (1)
  1. 22 August, 09:49
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    Option b. 6 h.

    Explanation:

    1) dа ta:

    a) A₀ = 80.0 mg

    b) A₁ = 5.0 mg

    c) t = 24.0 h

    2) Definition:

    Half-life is the time required to decrease the initial amount of a sample of matter by half.

    The half-life of each kind of radioisotope is a constant.

    3) Solution:

    The ratio of final mass to initial mass of the radioisotope is:

    5.0 mg / 80.0 mg = 1 / 16 = (1 / 2) ⁴ = (1/2) * (1/2) * (1/2) * (1/2)

    That shows that 4 half-lives have passed so that the isotope content has been reduced from 80.0 mg to 5 mg.

    Since, the half-life is a constant, each half - life is equal to

    24.0 h / 4 = 6 h, which is the option b.
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