Given: Ferric oxide has hexagonal close packed array.
Every three octahedral holes are occupied by ferric ions.
To find: Formula of the ferric oxide = ?
Calculation:
Let the number of oxide (O2− ) ions be x.
So, number of octahedral voids = x
It is given that two out of every three octahedral holes are occupied by ferric ions.
So, number of ferric (Fe3+) ions = 2/3x
Therefore, ratio of the number of Fe3+ ions to the number of O2− ions,
Hence, the formula of the ferric oxide is Fe2O3.