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Question

Iron(II)oxide crystallise in cubic structure with unit cell edge length of 5 A. If the density of oxide is 3.8 g cm3. Calculate the number of Fe2+ ions present in each unit cell.
[ Molar mass of Fe=56 g/mol ]

A
1
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B
2
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C
3
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D
4
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Solution

The correct option is D 4
Given:
Edge length, a= 5 A=5×108cm
Density, D = 3.8 g cm3
Molecular mass of FeO =56+16=72 u
We have the equation,
D=ZMNAa3
NA6.023×1023
ZNumber of FeO per unitcell
Substituting the values,
Z=DNAa3M=3.8×6.023×1023×(5×108)372=4
So there are four FeO per unit cell. Hence 4 Fe2+ per unit cell.


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