When heated above 916∘C, 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=1.73and√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 B0.917 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=4r√3
In FCC, a=4r√2 ∴dBCCdFCC=(2M×3√3/(NA×64r3))(4M×2√2/(NA×64r3))=0.917