Question

Iron exhibits bcc structure at room temperature. Above 900°C, it transforms to fcc structure.

Answer 1

Complete question is

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

For BCC,

4r = 1.73 a

a = 4 r /1.73 -----(1)

For FCC,

4 r = 1.4 a

a = 4 r / 1.4 ------(2)

For density, d = Z M / (a)3 NA

dBCC /dFCC = (Z /a3 )BCC /(Z / a3)FCC

= (2 /a3 )BCC /(4 /a3 )FCC

= 0.910

Thus, ratio of density of iron at room temperature to that at 900°C is 0.910

Answer 2

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