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Question
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.
Solution
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+ : = `O^(2-) = 2/3x:x`
`= 2/3 :1`
= 2 : 3
Hence, the formula of the ferric oxide is Fe2O3.
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