The Solid State - Result Question 6
####6. The pyknometric density of sodium chloride crystal is $2.165 \times 10^{3} kg m^{-3}$ while its X-ray density is $2.178 \times 10^{3} kg m^{-3}$. The fraction of unoccupied sites in sodium chloride crystal is
(a) $5.96 \times 10^{-3}$
(b) 5.96
(c) $5.96 \times 10^{-2}$
(d) $5.96 \times 10^{-1}$
[2003]
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Solution:
- (a) Fraction of unoccupied sites in $NaCl$ crystal $=\frac{x \text{-ray density-pyknometric density }}{x \text{-ray density }}$
$ =\frac{2.178 \times 10^{3}-2.165 \times 10^{3}}{2.178 \times 10^{3}}=\frac{0.013 \times 10^{3}}{2.178 \times 10^{3}} $
$=5.96 \times 10^{-3}$
Pyknometric density is nearest to true density calculated from the molecular mass and crystalline lattice of the product.
