When $NaCl$ (molten) is doped with $10^{-5} \, mol \, \%$ of $AlCl_3$,what is the concentration of cation vacancies?

Which one of the following compounds shows both,Frenkel as well as Schottky defects?

Calcium crystallises in a face-centred cubic unit cell with edge length $a = 0.556 \ nm$. Calculate the density. If it contained $0.1 \%$ Frenkel defects,what is the density in $g \ cm^{-3}$?

The correct statement among the following,regarding defects in solids is:
$(1)$ Frenkel defect is favored by small difference in the sizes of cation and anion.
$(2)$ Frenkel defect is a metal excess defect.
$(3)$ Trapping of electron in the lattice leads to formation of $F$-centres.
$(4)$ Schottky defect has no effect on the physical property of solids.

Identify the defect developed when an ion of an ionic compound is missing from its regular site and occupies an interstitial position between lattice points.

Analysis of the density of an electrically neutral $NaCl$ crystal containing vacancies yielded the following results:
$(i)$ Density of $NaCl$ crystal with all sites occupied $= 2.178 \times 10^3 \ kg \ m^{-3}$
$(ii)$ Density of $NaCl$ crystal with vacancies $= 2.165 \times 10^3 \ kg \ m^{-3}$
Then the percentage of vacancies in the $NaCl$ crystal is .....