An element (atomic mass 100 ~g / mol ) having BCC structure has...

Atomic mass of element $=100 \mathrm{~g} / \mathrm{mol}$ Cell edge $=400 \mathrm{pm}=400 \times 10^{-12}=4 \times 10^{-10}$ and number of atoms in $\mathrm{BCC}(Z)=2$. As the atomic mass of the metal is $100 \mathrm{~g} / \mathrm{mol}$, therefore mass of each atom $(\mathrm{m})=\frac{100}{6.023 \times 10^{23}}=16.6 \times 10^{-23} \mathrm{~g}$ The volume of unit cell $=\left(4 \times 10^{-8}\right)^3=64 \times 10^{-24} \mathrm{~cm}^3$. And mass of unit cell $=Z \times m=2 \times\left(16.6 \times 10^{-23}\right)=33.2 \times 10^{-23} \mathrm{~g}$. Therefore density of element $=\frac{\text{ Mass of unit cell }}{\text{ Volume of unit cell }}=\frac{33.2 \times 10^{-23}}{64 \times 10^{-24}} \approx$5.188 $\mathrm{~g}$/ \mathrm{cm}^3 $\text{. }$