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Question

When heated above 916C, iron changes its BCC crystalline form to FCC without the change in the radius of atom. The ratio of density of the crystal before heating and after heating is (Given : 3=1.73 and 2=1.41):

A
1.069
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B
0.917
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C
0.725
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D
1.231
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Solution

The correct option is B 0.917
Density=z × Ma3 × NA
where
z=number of atoms in the unit cellM=molecular massa=edge length of the unit cellNA= Avogardo's number
In BCC,
a=4r3
In FCC,
a=4r2
dBCCdFCC=(2M×33/(NA×64r3))(4M×22/(NA×64r3))=0.917

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