Ga (atomic mass $70 \mathrm{u}$ ) crystallizes in a hexagonal close packed structure. The total number of voids in $0.581 \mathrm{~g}$ of $\mathrm{Ga}$ is_______________ $\times 10^{21}$. (Round off to the Nearest Integer).
(15)
HCP structure: Per atom, there will be one octahedral void (OV) and two tetrahedral voids
(TV). Therefore total three voids per atom are present in HCP structure.
$\rightarrow$ therefore total no of atoms of Ga will be-
$=\frac{\text { Mass }}{\text { Molar Mass }} \times \mathrm{N}_{\mathrm{A}}=\frac{0.58 \mathrm{lg}}{70 \mathrm{~g} / \mathrm{mol}} \times 6.023 \times 10^{23}$
$\rightarrow$ Now, total Number of voids $=3 \times$ total no. of atoms
$=3 \times \frac{0.581}{70} \times 6.023 \times 10^{23}=14.99 \times 10^{21}$
$\simeq 15 \times 10^{21}$