Iron oxide FeO , crystallises in a cubic lattice with a unit cell...

The density of the crystal can be calculated as follows,

$d = \dfrac{Z \times M}{{N}_{0} \times {a}^{3}}$

$d = densityZ = effectivenumberofatomsM = Molarmass{N}_{0} = Avogadro ' snumbera = Edgelength$

$4 = \dfrac{Z \times 72}{6 \times {10}^{23} \times 125 \times {10}^{- 24}}$

$Z = 4 . 166 ≃ 4$