Answer
$1.99\times10^{25}$ atoms.
Work Step by Step
Edge length= 2.78 in= $2.78\,in\times\frac{2.54\,cm}{1\,in}=7.0612\,cm$
Volume of the cube= $(Edge\,length)^{3}=(7.0612\,cm)^{3}=352.075\,cm^{3}$
Mass= $Density\times Volume= 4.50 g/cm^{3}\times352.075\,cm^{3}=1584.34\,g$
Number of moles= $\frac{Mass\,in\,grams}{Molar\,mass}=\frac{1584.34g}{47.867\,g/mol}=33.0988\,mol$
Number of atoms= $33.0988\,mol\times\frac{6.022\times10^{23}}{1\,mol}=1.99\times10^{25}\,atoms$
