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 ) (given $:N_A=6.023\times {10}^{23}$)

Ga Crystallises in a hexagonal close packing
No. of  atoms per unit cell $=\frac{1}{6}\times 12+\frac{1}{2}\times 2+1\times 3=6$

No. of tetrahedral voids $=2\times 6=12$ No. of octahedral voids = 6

Total voids per unit cell = 18Total voids per atom of  $Ga=\frac{18}{6}=3$

Total no. of atoms of  $Ga=\frac{\text{ mass }}{\text{ molar mass }}\times N_a=\frac{0.581}{70}\times 6.023\times {10}^{23}$

Total no. of voids $=3\times \text{ total no. of atoms }=3\times \frac{0.581}{70}\times 6.023\times {10}^{23}$
$=14.99\times {10}^{21}\approx 15\times {10}^{21}$