Which of the following statements is true about schottky defect ?

In ionic solid, anions are arranged in hcp array and cations occupy $\dfrac{1}{2}$ tetrahedral voids. What is the formula of ionic compound ?
[Consider A = Cation ; B = anion]

Calculate the molality of aqueous solution of electrolyte that freezes at $-0.93\,^\circ\text{C}$ if $K_f$ for water and van't Hoff factor respectively are $1.86\,\text{K kg mol}^{-1}$ and $1.25$. (Freezing point of water $= 0\,^\circ\text{C}$ )

Calculate the solubility of certain gas in solvent with pressure $3$ atm at $25^\circ\text{C}$ (Henry's law constant is $3.0 \times 10^{-2}\,\text{mol dm}^{-3}\,\text{atm}^{-1}$)