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NCERT Class XII Chemistry
Chapter - The Solid State
Questions with Solutions

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Question : 19
Total: 50
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:  
Suppose the number of oxide ions (O‌2–) in the packing = 90
∴ Number of octahedral voids = 90
As 2/3‌rd of the octahedral voids are occupied by ferric ions, therefore,
number of ferric ions present =
2
3
×90=60
∴‌‌ Ratio of Fe3+:O2−=60:90=2:3
Hence, the formula of ferric oxide is Fe2O3.
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