Solid State

For HCP structure,
Effective number of atoms per unit cell (Z)=6
Number of tetrahedral void =2Z=12
Number of octahedral void =Z=6
Total number of voids per unit cell =18
Moles of Ga =‌

‌ Given mass ‌

‌ Molar mass ‌

=‌

0.581

70

No. of atoms =‌

0.581

70

×NA
No. of unit cells =‌

0.581

70

×‌

NA

6

No. of voids =(‌

0.581

70

×‌

6.02×1023

6

)×18=14.99×1021.
Nearest integer =15×1021
So, correct answer is 15 .