Answer 1

Given,

Mass of Al = 8.1

Atomic mass of Al = 27 g mol^-1

Number of atoms = number of moles × 6.022 × 10²³ = `8.1/27` × 6.022×10²³

= 0.3 × 6.022 × 10²³

= 1.8066×10²³

As one fcc unit cell have 4 atoms 

Hence number of unit cells having 1.8066×10²³ atoms = `(1.8066 xx 10^23)/4 = 4.51 xx 10^22` unit cell