Ga ( atomic mass 70 u) crystallizes in a hexagonal close packed structure. The total number of voids in 0.581 g of Ga is ____________$\times {10}^{21}$ .(Round off to the Nearest Integer)

Moles of Ga:

$$\begin{gathered} \mathrm{Moles}=\frac{\mathrm{Atomic} \mathrm{Mass}}{\mathrm{Gram} \mathrm{atomic} \mathrm{Mass}} \\ \mathrm{Moles}=\frac{0.581}{70} \end{gathered}$$

Number of Ga atoms:

$$\begin{gathered} \mathrm{No} \mathrm{of} \mathrm{atoms} \mathrm{of} \mathrm{Ga}=\frac{0.581}{70}\times {\mathrm{N}}_{\mathrm{A}} \\ =\frac{0.581}{70}\times 6.023\times {10}^{23}=0.0499\times {10}^{23} \mathrm{atoms} \end{gathered}$$

In hexagonal close packing, there are one octahedral void and two tetrahedral voids. 

$\begin{array}{l}\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{voids}=0.0499\times {10}^{23}\times 3=0.1499\times {10}^{23}\text{ voids}\cong 15\times {10}^{21} \mathrm{voids}\\ \mathrm{x}=15\end{array}$