why is ` Fe0(s)` not formed in stoichiometric compostion ?

Answer

This is because some `Fe^(2+)` ions in FeO are replaced by ` Fe^(3+)` ions. As ` 3 Fe^(2+)` ions are replaced by 2` Fe^(3+)` ions to maintain electrical neutality, there is less amount of metal as compared to stocihonmetric proportion .