Calculate the volume of fcc unit cell in $\mathrm{cm}^3$ if void volume of it is $4.16 \times 10^{-24} \mathrm{~cm}^3$.

Calculate the number of atoms present per unit cell if product of density and volume of unit cell is $1.8 \times 10^{-22} \mathrm{~g}$.

[Mass of an atom $=4.5 \times 10^{-23} \mathrm{~g}$ ]

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

Identify the defect that develops when the ratio of number of cations to anions become different from that indicated by its chemical formula.