Question

Potassium crystallises in BCC lattice. What is the number of unit cells in $3.9\text{g}$ of $K$.
(Atomic weight of $K=39u$)

• A. $N_A/20$
• B. $N_A/30$
• C. $N_A/40$
• D. $N_A/10$

Answer 1

The correct option is A $N_A/20$

$\text{Number of atoms},K=\frac{3.9}{39}\times N_A=\frac{N_A}{10}$
In BCC, $Z_{eff}=2$

$\text{Number of unit cells}=\frac{1}{2}\times \text{Number of atoms}$
$\Rightarrow \text{Number of unit cells}=\frac{1}{2}\times \frac{N_A}{10}$
$\Rightarrow \text{Number of unit cells}=\frac{N_A}{20}$