Ncert Exercises & Solutions

1.19 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.

Let the number of oxides (O²⁻) ions be x.
So, the 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

= \frac{2}{3} x

Therefore, the ratio of the number of Fe³⁺ ions to the number of O²⁻ ions,

{Fe}^{3 +} : O^{2 -} = \frac{2}{3} x : x

= \frac{2}{3} : 1

= 2 : 3

Hence, the formula of the ferric oxide is Fe₂O₃.

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