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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.
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Solution
In hexagonal close packing (hcp), the quantity of octahedral voids is equivalent to the number of constituent particles.
Fe3+ ions occupy two thirds of the octahedral voids in ferric oxide, while O2+ ions form the cubic close-packed structure. If there are nO2−ions in the unit cell, the quantity of Fe3+ ions per unit cell would be `(2 n)/3`.
Hence, formula of ferric oxide = `((2 n)/3)` Fe3+ . (n) O2−
= Fe2nO3n
= Fe2O3
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