What is minimum number of spheres required of develop a tetrahedral void?

Calculate the volume of unit cell if an element having molar mass $92 \mathrm{~g} \mathrm{~mol}^{-1}$ that forms bcc structure $\left[\varrho \times \mathrm{N}_{\mathrm{A}}=5.0 \times 10^{24} \mathrm{~g} \mathrm{~cm}^{-3} \mathrm{~mol}^{-1}\right]$

In an ionic solid equal number of cations and anions are missing from their regular positions in the crystal lattice creating vacancies is called-

Calculate number of atoms per unit cell of an element having molar mass $92.0 \mathrm{~g} \mathrm{~mol}^{-1}$ and density $8.6 \mathrm{~g} \mathrm{~cm}^{-3}$ forming cubic unit cell structure. $\left[\mathrm{a}^3 \times \mathrm{N}_{\mathrm{A}}=21.5 \mathrm{~cm}^3 \mathrm{~mol}^{-1}\right]$