Ferric oxide crystallizes 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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Unit cell of hcp structure = 6 No. of oxide ions per unit cell = 6 No. of octahedral voids = 6 Since, ferric ions occupy only two out of every three octahedral voids, Therefore, no. of octahedral holes occupied by ferric ions = 2/3 X 6 = 4 Stoichiometric ratio of Fe and O = 4:6 = 2:3 Hence, formula of ferric oxide = Fe2O3.

 

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