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Question:

A compound is formed by cation C and anion A. The anions form a hexagonal close packed (hcp) lattice and the cations occupy 75% of the octahedral voids.

C2A3

C3A2

C3A4

C4A3

Solution:

Correct option is C. C3A4
Anions (A) are in hcp, so the number of anions (A) = 6.
Cations (C) are in 75% of octahedral voids. In hcp, the number of octahedral voids is equal to the number of anions. Therefore, the number of octahedral voids is 6.
Number of cations (C) = 75% of 6 = (75/100) * 6 = 4.5 ≈ 4.5 (Since we cannot have a fraction of an atom)
The ratio of cations (C) to anions (A) is 4.5:6, which simplifies to 3:4.
Therefore, the formula of the compound is C3A4.