A metal crystallizes in bcc lattice with unit cell edge length of 300pm and density $6.15 {\mathrm{gcm}}^{-3}.$ The molar mass of the metal is 

$$\begin{gathered} \mathrm{d}=\frac{\mathrm{Z}\times \mathrm{M}}{{\mathrm{a}}^3\times {\mathrm{N}}_{\mathrm{A}}}\Rightarrow \mathrm{M}=\frac{{\mathrm{da}}^3\mathrm{NA}}{\mathrm{Z}},\mathrm{for}\mathrm{bcc},\mathrm{z}=2 \\ \begin{array}{l}\Rightarrow \frac{6.15\times {(300\times {10}^{-3})}^3\times 6.02\times {10}^{23}}{2}\\ =50{\mathrm{gmol}}^{-1}\end{array} \end{gathered}$$