A metal $\mathrm{M}$ forms hexagonal close-packed structure. The total number of voids in $0.02 \mathrm{mol}$ of it is _____ $\times {10}^{21}$ (Nearest integer)

(Given ${\mathrm{N}}_{\mathrm{A}}=6.02\times {10}^{23}$)

From the diagram of voids in HCP:

There are 6 octahedral voids and 12 tetrahedral voids in a HCP unit cell.
One unit cell of HCP contains $=18$ voids

Thus, No. of voids in $0.02 \mathrm{mol}$ of hcp

$=\frac{18}{6}\times 6.02\times {10}^{23}\times 0.02$

$\approx 3.6\times {10}^{22}$

$\approx 36\times {10}^{21}$