A compound forms hexagonal closed packed structure. What is the number of tetrahedral voids in 0.8 mol of it?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(9.635 × 10^{23}\)
In a close-packed structure (hcp or ccp) there are two tetrahedral voids and one octahedral void for every atom (sphere) in the packing.
Number of atoms in 0.8 mol:
\( N = 0.8 \times 6.022 \times 10^{23} = 4.818 \times 10^{23} \)
Number of tetrahedral voids:
\( 2N = 2 \times 4.818 \times 10^{23} = 9.635 \times 10^{23} \)
There are 9.635 × 1023 tetrahedral voids.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Formula of a Compound and Number of Voids Filled – “Earlier in the section, we have learnt that when particles are close-packed resulting in either ccp or hcp structure, two types of voids are generated. While the number of octahedral voids present in a lattice is equal to the number of close packed particles, the number of tetrahedral voids generated is twice this number.”