Answer
$2.79\times10^{-3} s^{-1}$
Work Step by Step
Recall that $t_{1/2}=\frac{0.693}{k}$ or $k=\frac{0.693}{t_{1/2}}$.
That is, $k=\frac{0.693}{248 s}=2.79\times10^{-3}s^{-1}$
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