$\begin{aligned} & \mathrm{d}=\frac{\mathrm{ZM}}{\mathrm{N}_{\mathrm{A}} \cdot \mathrm{a}^3} \\ & \mathrm{a}=400 \mathrm{pm} \\ & =400 \times 10^{-10} \mathrm{~cm} \\ & =4 \times 10^{-8} \mathrm{~cm} \\ & \mathrm{Z}=2 \text { for BCC } \\ & 4=\frac{2 \times \mathrm{M}}{6.022 \times 10^{23} \times\left(4 \times 10^{-8}\right)^3} \\ & \mathrm{M}=\frac{4 \times 6.022 \times 10^{23} \times 64 \times 10^{-24}}{2} \\ & =77 \mathrm{~g} \mathrm{~mol}^{-1}\end{aligned}$