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Question: Answered & Verified by Expert
A metal oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by metal ions. The formula of metal oxide is
ChemistrySolid StateAP EAMCETAP EAMCET 2018 (23 Apr Shift 2)
Options:
  • A $M O$
  • B $M_3 \mathrm{O}_4$
  • C $M_2 \mathrm{O}_5$
  • D $M_2 \mathrm{O}_3$
Solution:
2176 Upvotes Verified Answer
The correct answer is: $M_2 \mathrm{O}_3$
Unit cell of hcp structure $=6$ number of oxide ions per unit cell $=6$ number of octahedral voids $=6$. Since, ferric ions occupy only two out of every three octahedral voids, number of octahedral holes occupied by ferric ions $\frac{2}{3} \times 6=4$ stoichiometric ratio of metal and ion $=4: 6=2: 3$.
Hence, formula of metal oxide is $M_2 \mathrm{O}_3$.

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