An element 'X' (At. mass=40 g/mol) having f.c.c structure, has unit cell edge length of 400 pm. Calculate the density of 'X' and the number of unit cells in 4g of 'X'. (NA=6.022×10²³ mol⁻¹)

edge length = 400 pm= 400 ×10⁻¹⁰ cm= 4 ×10⁻⁸ cm
Density = Atomic mass × Number of atoms in 1 unit cell / Avogadro's number ×( edge length)³
Density =40 g/mol × 4 / 6.022 ×10²³ mol⁻¹ ×( 4 ×10⁻⁸ cm)³
Density =4.15 g/cm³
Volume of 4 g of 'X'= 4 g / 4.15 g/cm³=0.964 cm³
Volume of one unit cell=( edge length)³=( 4 ×10⁻⁸ cm)³=6.4×10⁻²³ cm³
The number of unit cells in 4g of 'X'=0.964 cm³ / 6.4×10⁻²³ cm³=1.5×10²²