1.19 Ferric oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.
Let the number of oxide ions be x.
Then, the number of octahedral voids = number of oxide ions = x.
According to question, number of ferric ions
Ratio of ferric ion: oxide ion = 2:3
So the required formula of the compound is Fe2O3.