Question

How many unit cells are present in $5.0$ g of crystal AB (formula mass of AB $=40$) having rock salt type structure? ($N_A$= Avogadro's number)

• A. $N_A$
• B. $\frac{N_A}{10}$
• C. $4N_A$
• D. None of the above

Answer 1

The correct option is D None of the above

In rock salt type structure there are $4$ $AB$ molecules in one unit cell.
In $5.0g$ of crystal $AB$ there are $\frac{5}{40}\times N_A$ molecules and $\frac{5}{40}\times N_A\times \frac{1}{4}$ unit cells.
So, the number of unit cells is $\frac{1}{32}{N}_A$.