Answer
$5.1\times10^{24}\,amu$
Work Step by Step
1 amu=$1.66\times10^{-24}\,g$
$\implies 8.4\,g=8.4\,g\times\frac{1\,amu}{1.66\times10^{-24}\,g}$
$=5.1\times10^{24}\,amu$
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