Q13. (i) Calculate the number of unit cells present in 58.5 g of NaCl (Na = 6 x 1023) crystal lattice.
(ii) Write any two characteristics features of Frenkel defect.
Answers
Explanation:
The molar mass of NaCl is ⇒23+35.5=58.5 g.
Thus, 58.5 g of NaCl corresponds to 1 mole of NaCl and contains 6.023×10
23
molecules.
One unit cell of NaCl (fcc structure) has 4 NaCl molecules. Hence, 1 mole of NaCl will contain
4
6.023×10
23
≈1.5×10
23
unit cells.
The number of unit cells in 58.5 g of NaCl is nearly 1.5×10
23
2.Frenkel Defects: This defect arise when an ion occupies an interstitial position between the lattice points. This defect occurs generally in ionic crystals in which the anion is much larger in size than the cation.
AgBr is an example for this type of defect. One of the Ag
+
ion occupies a position in the interstitial space rather than its own appropriate site in the lattice.
The crystal remains neutral since the number of positive ions is the same as the number of negative ions.
may it helps you dear
(i) answer of this is above in the attachment.
(ii)
This defect occurs only when cations are smaller when compared to the anions. There are also no changes in chemical properties. There is also no significant impact of Frenkel defect on the density of the solid and therefore both the volume and mass of the solid is preserved. In such cases, substances also maintain their electrical neutrality.
