Question

How many unit cells are present in a cubic shaped ideal crystal of NaCl of mass $1.0$ g?

• A. $1.28\times {10}^{21}$
• B. $1.71\times {10}^{21}$
• C. $2.57\times {10}^{21}$
• D. $5.14\times {10}^{21}$

Answer 1

Answer: (C)

$2.57\times {10}^{21}$

In $1.0$ g of $NaCl$, there are $\frac{1}{58.5}$ moles $=\frac{1}{58.5}\times 6\times {10}^{23}$ molecules
There are 4 molecules of $NaCl$ per unit cell of crystal.
Therefore, the number of unit cell $=\frac{6\times {10}^{23}}{4\times 58.5}$ $=2.57\times {10}^{21}$.