Let us Suppose the number of oxide (O2–) ions = N.
Number of octahedral void = number of anions
So that the number of octahedral voids = N
We have Given that
Two out of every three octahedral holes are occupied by ferric ions.
So that the number of ferric (Fe3+) ions = 2N/3
The ratio of the number of Fe3+ ions to the number of O2− ions,
Fe3+ : O2− = 2N/3 : N
Multiply 3 and divide by N we get
Fe3+ : O2− = 2 : 3
Hence, the formula of the ferric oxide is Fe2O3.