KVPY Sample Paper KVPY Stream-SX Model Paper-15

  • question_answer
    A metal crystallises into two cubic phases, face centred cubic (fee) and body centred cubic (bcc), whose unit cell lengths are 3.5 and \[3.0\overset{{}^\circ }{\mathop{A}}\,\] respectively. Calculate the ratio of densities of fee and bcc.

    A) 1.26

    B) 3.25    

    C) 7.8

    D) 5.35

    Correct Answer: A

    Solution :

    Density \[=\frac{M.Z}{{{N}_{A}}\times {{a}^{3}}}\]Density \[\propto \frac{Z}{{{a}^{3}}}\]
    In case of fcc, \[Z=4,\]\[a=3.5\overset{{}^\circ }{\mathop{A}}\,\]
    \[{{d}_{1}}=\frac{4}{{{(3.5)}^{3}}}\]
    In case of bcc \[Z=2,a=3\overset{{}^\circ }{\mathop{A}}\,\]
    \[{{d}_{1}}=\frac{2}{{{(3)}^{3}}}\]
    \[\Rightarrow \]\[\frac{{{d}_{1}}}{{{d}_{2}}}=\frac{{{N}_{1}}}{{{N}_{2}}}\left( \frac{{{a}_{2}}}{{{a}_{1}}} \right)=\frac{4}{2}{{\left( \frac{3}{3.5} \right)}^{3}}=1.26\]


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