A) Four \[F{{e}^{2+}}\] and four \[{{O}^{2-}}\]
B) Two \[F{{e}^{2+}}\] and four \[{{O}^{2-}}\]
C) Four \[F{{e}^{2+}}\] and two \[{{O}^{2-}}\]
D) Three \[F{{e}^{2+}}\] and three \[{{O}^{2-}}\]
Correct Answer: A
Solution :
Let the units of ferrous oxide in a unit cell \[=n\], molecular weight of ferrous oxide \[(FeO)=56+16=72\,g\,mo{{l}^{-1}},\] weight of n units \[=\frac{72\times n}{6.023\times {{10}^{23}}}\] Volume of one unit \[={{(\text{length}\,\text{of}\,\text{corner})}^{3}}\] \[={{(5{\AA})}^{3}}=125\times {{10}^{-24}}c{{m}^{3}}\] Density\[=\frac{\text{wt}\text{.}\,\text{of}\,\text{cell}}{\text{volume}},\]\[4.09=\frac{72\times n}{6.023\times {{10}^{23}}\times 125\times {{10}^{-24}}}\] \[n=\frac{3079.2\times {{10}^{-1}}}{72}=42.7\times {{10}^{-1}}=4.27\approx 4\]
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