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,
Fe3+ : O2- = 2/3 x : x
Hence, the formula of the ferric oxide is Fe2O3.