Recall that ** half-life** is the time needed for the amount of a reactant to decrease by 50% or one-half.

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

$\overline{){{\mathbf{t}}}_{\raisebox{1ex}{$\mathbf{1}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{2}$}\right.}{\mathbf{=}}\frac{\mathbf{ln}\mathbf{2}}{\mathbf{k}}}$

where:

**t**_{1/2} = half-life

**k** = decay constant

The bromine-82 nucleus has a half-life of 1.0 X 10 ^{3} min. If you wanted 1.0 g ^{82}Br and the delivery time was 3.0 days, what mass of NaBr should you order (assuming all of the Br in the NaBr was ^{82}Br)?

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