When heated above 916∘C, iron changes its BCC crystalline form to FCC without any change in the radius of atom. The ratio of density of the crystal before heating and after heating is [atomic weight of Fe=56]
A
1.069
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B
0.918
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C
0.725
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D
1.231
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Solution
The correct option is B 0.918 We know that,
Density = d =Z×MMa3×NA
The ratio of density of the crystal before heating and after heating is d1d2=ZBCC×MMa3BCC×NAZFCC×MMa3FCC×NA =ZBCCa3BCCa3FCCZFCC =2(4r√3))3×(2√2r)34
= 0.918