Example

Try It 10.89: Evaluating an Exponential Decay Model Using Half-Life

Practice solving an exponential decay problem using half-life. Suppose the half-life of magnesium-27 is 9.459.45 minutes, and we want to find how much of an initial 1010-mg sample remains after 66 minutes. Using the exponential decay formula A=A0ektA = A_0 e^{kt}, first determine the decay rate kk by recognizing that half the sample (55 mg) remains after 9.459.45 minutes: 5=10ek9.455 = 10 e^{k \cdot 9.45}. Dividing by 1010 gives 0.5=e9.45k0.5 = e^{9.45k}. Taking the natural logarithm of both sides yields ln0.5=9.45k\ln 0.5 = 9.45k, so k=ln0.59.45k = \frac{\ln 0.5}{9.45}. To find the amount remaining after 66 minutes, substitute this rate and time into the formula: A=10eln0.59.456A = 10 e^{\frac{\ln 0.5}{9.45} \cdot 6}. Evaluating this expression yields approximately 6.446.44 mg. Thus, about 6.446.44 mg of the sample will be left.

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Updated 2026-05-26

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