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 .