Iron oxide FeO, crystallises in a cubic lattice with a unit cell edge length of 5.0 $\AA$. If densit

Question

Iron oxide FeO, crystallises in a cubic lattice with a unit cell edge length of 5.0 $\AA$. If density of the FeO in the crystal is 4.0 g $cm^{-3}$ then the number of FeO units present per unit cell is (Nearest integer).

Professor PC Thomas Classes · Answer

Using $Z = \frac{d \cdot N_A \cdot a^3}{M_{FeO}} = \frac{4.0 	imes 6.02 	imes 10^{23} 	imes (5 	imes 10^{-8})^3}{72} = \frac{4.0 	imes 6.02 	imes 125 	imes 10^{-1}}{72} \approx 4.18$. Rounding to the nearest integer gives 4.