A) two \[F{{e}^{2+}}\] and four \[{{O}^{2-}}\]
B) three \[F{{e}^{2+}}\] and three \[{{O}^{2-}}\]
C) four \[F{{e}^{2+}}\] and two \[{{O}^{2-}}\]
D) four \[F{{e}^{2+}}\] and four \[{{O}^{2-}}\]
Correct Answer: D
Solution :
| [d] Let the units of ferrous oxide in a unit cell = n |
| Molecular weight of ferrous oxide \[(FeO)\] |
| \[=56+16=72\text{ }g\text{ }mo{{l}^{-1}}\] |
| Weight of n units \[=\frac{72\times n}{6.023\times {{10}^{23}}}\] |
| Volume of one unit \[\text{= (length of corner}{{\text{)}}^{\text{3}}}\] |
| \[={{(5{{A}^{0}})}^{3}}=125\times {{10}^{-24}}c{{m}^{3}}\] |
| \[\text{Density =}\frac{\text{wt}\text{.of cell}}{\text{volume}}\] |
| \[\therefore \] \[4.09=\frac{72\times n}{6.023\times {{10}^{23}}\times 125\times {{10}^{-24}}}\] |
| Hence, \[n=\frac{3079.2\times {{10}^{-1}}}{72}=42.7\times {{10}^{-1}}\] |
| \[=4.27=4\] |
| [d] They are comparatively soft and not very rigid. |
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