We’re being asked **to determine the time** it takes for 12.0 μg of Chromium–51 to **decrease** to 1.50 μg.

Recall that ** radioactive/nuclear decay of isotopes** follows first-order kinetics, and the integrated rate law for first-order reactions is:

where:

**[N] _{t}** = concentration at time t

**k** = decay constant

**t** = time

**[N] _{0}** = initial concentration.

The half-life of Chromium-51 is 28 days. Starting with an original sample size of 12.0μg, calculate the number of days it will take for the radioactive isotope to drop to 1.50μg.

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