A diatomic molecule   has a Body-Centered Cubic (BCC)  structure with a cell edge of 300 Pm. The density of , molecule is  . The number of molecules present in 200 g of    is:

(Avogadro constant  

Atom X are present in bcc lattice. For atom X, effective number of atoms in the units cell=2For molecule $$\begin{gathered} X_2, \\ Z=2,d=\frac{z\times M}{N_A\times a^3}\Rightarrow \\ 6.17=\frac{2\times M}{6\times {10}^{23}\times {\left(300\times {10}^{-10}\right)}^3} \end{gathered}$$ 
$\Rightarrow M=50\text{g}/\text{mol}$ . Number of molecules present in 200g of $X_2=\frac{200\text{g}}{50\text{g}/\text{mol}}\times N_A$  molecules/mol