Question
Ferric oxide crystallizes in a hexagonal close packed array of oxide ions with two out of every three tetrahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.
Solution
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.
