RAJASTHAN ­ PET Rajasthan PET Solved Paper-2012

  • question_answer
    \[CsBr\]crystallises in a body centred cubic lattice. The unit cell length is 436.6 pm. Given that the atomic mass of Cs = 133 u and that of Br = 80 u and Avogadro number being \[6.0.2\times {{10}^{23}}mo{{l}^{-1}},\] the density of\[CsBr\]is

    A)  \[42.5g/c{{m}^{3}}\]  

    B)  \[0.425g/c{{m}^{3}}\]

    C)  \[8.25\text{ }g/c{{m}^{3}}\] 

    D)  \[4.25\text{ }g/c{{m}^{3}}\]

    Correct Answer: D

    Solution :

     Density of \[CsBr=\frac{Z\times M}{{{a}^{3}}\times {{N}_{0}}}\] where \[Z=\]no. of atoms in the bcc unit cell\[=2\] \[M=\]molar mass of\[CsBr=133+80=213\] \[a=\]edge length of unit cell = 436.6 pm \[=436.6\times {{10}^{-10}}cm\] \[\therefore \]Density \[=\frac{2\times 213}{{{(436.6\times {{10}^{-10}})}^{3}}\times 6.02\times {{10}^{23}}}\] \[=8.50g/c{{m}^{3}}\] For a unit cell density\[=\frac{8.50}{2}=4.25\text{ }g/c{{m}^{3}}\]


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