SOLUTION: Suppose you begin with 2.8 mg of a pure radioactive substance and 3.6 h later determine that 0.35 mg remain. What is the half-life of the substance?

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Solution:As we know the radioactive decay is given by:

where M is th actual mass of the substance, Mo is the mass after t= 3.6 h,
and k is decay coefficient. Substitute our values:
, Take ln for both sides.

k=-0.212 Now that, we know the coefficient of half-life, can compute the half-life.
, Take ln for both sides.

h.