The number of unit cells present in 39g of potassium if it...

Potassium has bcc system
Number of mole $\text{=}\text{39/39}\text{=}\text{1}$ mole
$\text{1}$ mole of atoms = \text{6}\text{.022} \times {\text{10}^{23} atoms $\text{=}\text{N}$
$∵$ In bcc system $\text{2}$ atoms are present in $\text{1}$ unit cell.

$∴ \text{N}$ number of atoms are present in $\dfrac{N}{2}$ unit cells.