Answer
$2.656\times10^{-23} g$
Work Step by Step
Since $1\ u = 1.66054\times10^{-24}\ g$:
The mass of $^{16}O$ in g is $15.995\ u \times 1.66054\times10^{-24} g/u = 2.6560\times10^{-23} g$
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