How many unit cells are present in $16.0gm$ of crystal $AB$ (formula mass of $AB=40$) having rock salt type structure.

[$N_A=$Avogadro's no.]

The correct option is B $0.1N_A$

In rock salt type structure there are 4 AB molecules in one unit cell.

In 16.0g of crystal AB there are $\frac{16}{40}\times N_A$ molecules and $\frac{16}{40}\times N_A\times \frac{1}{4}$ unit cells.

So, the number of unit cells is $0.1\times N_A$.