Ferrous oxide `(FeO)` crystal has a cubic structure and each edge of the unit cell is `5.0 A^(@)` Taking density of the oxide as `4.0gcm^(-3)`. The number of `Fe^(2+)` and `O^(2-)` ions present in each unit cell is:

Explanation

Volume of the unit cell `= (5xx10^(-8)cm)^(3)=1.25xx10^(-22)cm^(3)`
Density of `FeO = 4 g cm^(-3)`
Therefore, mass of unit cell `= 1.25xx10^(-22)xx4=5xx10^(-22)g`
Mass of one molecule of FeO = Molar mass in g/ Avogadro.s number
`= (72 g mol^(-1))/(6.022xx10^(23)mol^(-1))=1.195xx10^(-22)g`.
Number of FeO molecules per unit cell `= (5xx10^(-22))/(1.195xx10^(-22))~~4`