Question

How many unit cells are present in a cube-shaped ideal crystal of $NaCl$ of mass $1.00g$ [Atomic masses : $Na=23,Cl=35.5]$.

• A. $1.21\times {10}^{21}$ unit cells
• B. $5.48\times {10}^{21}$ unit cells
• C. $2.57\times {10}^{21}$ unit cells
• D. $7.42\times {10}^{21}$ unit cells

Answer 1

The correct option is C $2.57\times {10}^{21}$ unit cells

moles of $NaCl=\frac{1}{58.5}$ moles

No. of $NaCl$ molecule=$\frac{1}{58.5}\times 6.023\times {10}^{23}$ molecules

There are $4$ molecules of $NaCl$ present in an unit cell

No. of unit cell=$\frac{6.023}{58.5\times 4}\times {10}^{23}=2.57\times {10}^{21}$ unit cells.