AP EAMCET · Chemistry · Solid State

Iron exhibits bcc structure at room temperature. Above \(500^{\circ} \mathrm{C}\), it transforms to fcc structure. Find the ratio of the density of iron at room temperature to that at \(500^{\circ} \mathrm{C}\). (Assume the atomic radii and the molar mass of iron remain constant even with variation in temperature)

A \(3 \sqrt{3}: 4 \sqrt{2}\)
B \(\sqrt{3}: \sqrt{2}\)
C \(\sqrt{2}: \sqrt{3}\)
D \(10: 92\)

Verified Solution

Answer & Solution

Correct Answer

(A) \(3 \sqrt{3}: 4 \sqrt{2}\)

Step-by-step Solution

Detailed explanation

We know, Density, \(d=Z M / a^3 N_0\) For bcc unit cell, \(Z=2, a=\frac{4}{\sqrt{3}} r\) \[ d_{\mathrm{bcc}}=\frac{2 M}{\left(\frac{4}{\sqrt{3}} r\right)^3 N_0} \] For fcc unit cell, \(\quad Z=4, a=\frac{4}{\sqrt{2}} r\)…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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