A solid compound is formed by atoms of $A$ (cations), $B$ (cations) and O (anions). Atoms of O form hcp lattice. Atoms of $A$ occupy $25 \%$ of tetrahedral holes and atoms of $B$ occupy $50 \%$ octahedral holes. What is the molecular formula of solid?
Some substances are given below
$$ \begin{aligned} & \mathrm{Ag}, \mathrm{CO}_2(s), \mathrm{SiO}_2, \mathrm{ZnS} \\ & \mathrm{SO}_2(s), \mathrm{AlN}, \mathrm{HCl}(\mathrm{~s}), \mathrm{H}_2 \mathrm{O}(\mathrm{~s}) \end{aligned} $$
The number of molecular solids and network solids in the above list is respectively.
The diffraction pattern of crystalline solid gave a peak at $20=60^{\circ}$. What is the distance ( in cm ) between the layers which gave this peak?
( $\lambda$ of $X$-rays is $1.54 \mathring{A}$ ) $\left(\sin 30^{\circ}=0.5, \sin 60^{\circ}=0.866 ; n=1\right)$
(a) $8.89 \times 10^{-8}$