Ferrous oxide (FeO) crystal has a cubic structure and each edge of the unit cell is 5.0 Ao. Taking density of the oxide as 4.0 g cm-3. The number of FeO formula units present in each unit cell is:

Volume of the unit cell, a3=5×10−83=1.25×10−22cm3 (∵a=5Å )  Density (ρ)=4.0gcm−3 Mass of unit cell =V×ρ=1.25×10−22×4.0=5.0×10−22g Mass of one molecule of FeO= Mol. mass (in grams)  Avogadro's number =726.022×1023=1.195×10−22g Number of FeO molecules per unit cell =5.0×10−22g1.195×10−22g=4.19≈4Thus, there are four FeO formula units in each unit cell.