Solution:

a. The number of atoms in the close-packing is 1 mole i.e., 1 Ã— 6.022 Ã— 1023 = 6.022 Ã— 1023 atoms

In closed-packing:

Number of octahedral voids = Number of atoms = 6.022 Ã— 1023

Number of tetrahedral voids = 2 Ã— Number of atoms in the close-packing

= 2 Ã— 6.022 Ã— 1023

= 12.044 Ã— 1023

Thus, total number of voids =6.022 Ã— 1023 + 12.044 Ã— 1023 = 18.066 Ã— 1023

Hence, the total number of voids in 1 mole of a compound forming hexagonal close-packed structure is 18.066 Ã— 1023.

b. In a molecule of CaF2, the coordination number of Ca2+is 8 and that of Fâˆ’is 4.