A metal of molar mass 56 g/mol crystallises in a bcc structure with density 7.86 g/cm³. The unit cell edge length $a$ is approximately:
A2.86 Å
B2.86 × 10⁻⁸ cm
CBoth A and B (same value, different units)
D5.72 Å
Answer & Solution
Correct answer: C. Both A and B (same value, different units)
$a^3 = nM/(\rho N_A) = 2 \times 56/(7.86 \times 6.022\times10^{23}) = 2.37\times10^{-23}$ cm³. $a = (2.37\times10^{-23})^{1/3} \approx 2.87 \times 10^{-8}$ cm = 2.87 Å. This matches α-Fe (BCC iron).
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