The half-life of a first-order reaction is given by:

$\overline{){{\mathbf{t}}}_{\mathbf{1}\mathbf{/}\mathbf{2}}{\mathbf{=}}\frac{\mathbf{ln}\mathbf{2}}{\mathbf{k}}}$

We first need to calculate for the decay constant using the given half-life, **2.86 yrs**:

Plutonium-236 is an alpha emitter with a half-life of 2.86 years. How long will it take for the 1.30 mg sample of Pu-236 to decay to 0.160 mg?

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