The density and edge length values for a crystallise elemens with fcc lattice are 10g cm−3 and 400 pm, respectively. The number of unit cell in 32g of this crystal is:
Theoretical density of crystal, ρ=nMNoa3 g/cm3
Given that for fcc, n=4, a=400×10−10 cm, No=6×1023 ρ=10 g/cm3
∴M=ρ . No . a3n
=10×6×1023×(400)3×10−302
⇒M=96.4 g
Number of moles in 32 g of crystal =3296.4=0.33
Number of atoms =0.33×6.022×1023
Now, in fcc, there are 4 atoms in a unit cell.
So, number of unit cells =0.334×6.022×1023=5×1022