NEET Chemistry NEET PYQ-The Solid State

  • 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 and that of Br = 80 amu and Avogadro number being \[6.02\times {{10}^{23}}\]\[mo{{l}^{-1}}\], the density of CsBr is:[AIPMT (S) 2006]

    A) \[42.5\text{ }g/c{{m}^{3}}\]

    B) \[0.425\text{ }g/c{{m}^{3}}\]

    C) \[8.25\text{ }g/c{{m}^{3}}\]

    D) \[4.25\text{ }g/c{{m}^{3}}\]

    Correct Answer: D

    Solution :

    [d] Density of CsBr = \[\frac{Z\times M}{{{a}^{3}}\times {{N}_{0}}}\]
                Z\[\to \] no. of atoms in the bcc unit cell = 2
                M\[\to \]molar mass of CsBr = 133 + 80 = 2
                a \[\to \] edge length of unit cell
                = 436.6 pm
                = 436.6 × 10-10 cm
                \[\therefore \,\,\text{Density}\,=\frac{2\times 213}{{{(436.6\times {{10}^{-10}})}^{3}}\times 6.02\times {{10}^{23}}}\]
                = 8.50 g/cm3
                For a unit cell = \[\frac{8.50}{2}=4.25\,g/c{{m}^{3}}\]


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