4AB › Chemistry PYQ › CSIR-NET › December 2013 › Q112Q112 · CSIR-NET Chemistry, December 2013 Paper: CSIR-NET December 2013 · Subject: Physical Chemistry · Chapter: Solid State Chemistry · Topic: Crystal Systems Lattices · Marks: 2 · Difficulty: Medium
An element exists in two crystallographic - modifications with FCC and BCC structures. The ratio of the densities of the FCC and BCC modifications in terms of the volumes of their unit cells $\left(\mathrm{V}_{\mathrm{FCC}}\right.$ and $\left.\mathrm{V}_{\mathrm{BCC}}\right)$ is
(a) $\mathrm{V}_{\mathrm{BCC}}: \mathrm{V}_{\mathrm{FCC}}$
(b) $\mathrm{V}_{\mathrm{BCC}}: 2 \mathrm{V}_{\mathrm{FCC}}$
(c) $2 \mathrm{V}_{\mathrm{BCC}}: \mathrm{V}_{\mathrm{FCC}}$
(d) $\mathrm{V}_{\mathrm{BCC}}: \sqrt{2} \mathrm{V}_{\mathrm{FCC}}$
Answer Answer: C ✓ checked by 4AB · confidence high
The source book printed A; on checking, C is correct — see the explanation.
Printed A ignores that fcc holds 4 atoms per cell and bcc 2.
Explanation ρ = ZM/(N_AV): ρ_fcc/ρ_bcc = (4/V_fcc)/(2/V_bcc) = 2V_bcc : V_fcc.
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