$\begin{aligned} & \mathrm{a}=500 \mathrm{pm}=500 \times 10^{-10} \mathrm{~cm} \\ & =5 \times 10^{-8} \mathrm{~cm} \\ & \mathrm{Z}=2(\text { for } \mathrm{BCC}) \\ & \mathrm{d}=\frac{\mathrm{ZM}}{\mathrm{N}_{\mathrm{A}} \cdot \mathrm{a}^3} \\ & \mathrm{M}=\frac{\mathrm{d} \times \mathrm{N}_{\mathrm{A}} \times \mathrm{a}^3}{\mathrm{Z}}=\frac{4 \times 6.022 \times 10^{23} \times\left(5 \times 10^{-8}\right)^3}{2} \\ & \mathrm{M}=150 \mathrm{~g} \mathrm{~mol}^{-1}\end{aligned}$