#### 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 (O^{2−}) 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 (Fe^{3+}) ions`=2/3x`

Therefore, ratio of the number of Fe^{3+} ions to the number of O^{2−} ions,

Fe^{3+ }: = `O^(2-) = 2/3x:x`

`= 2/3 :1`

= 2 : 3

Hence, the formula of the ferric oxide is Fe_{2}O_{3}.

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Solution 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. Concept: Concept of Close Packed Structures.