Iron exhibits bcc structure at room temperature. Above 900°C, it transforms to fcc structure. The ratio of density of iron at room temperature to that at 900°C (assuming molar mass and atomic radii of iron remains constant with temperature) is
- 3 2
- 4 33 2
- 3 34 2
- 12
Answer
(3) 3 34 2
Solution
Density d = Z Ma3 NA ⇒ d Za3 (M and NA constant)
bcc: Z = 2, a = 4r 3
fcc: Z = 4, a = 2 2 r
⇒ dbccdfcc = 2 ( 4r 3 )3 (2 2 r )34
= 2 3 364 16 24
= 3 3 28
= 3 34 2
Answer: (3)