KVPY Sample Paper KVPY Stream-SX Model Paper-29

  • question_answer
    Which of the following expression is correct for packing fraction of \[NaCl\] if the ions along the face are diagonally removed?

    A) \[\frac{\frac{13}{3}\pi \operatorname{r}_{-}^{3}+\frac{16}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}+{{\operatorname{r}}_{-}} \right)}^{3}}}\]

    B) \[\frac{\frac{13}{3}\pi \operatorname{r}_{-}^{3}+\frac{4}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}+{{\operatorname{r}}_{-}} \right)}^{3}}}\]

    C) \[\frac{\frac{16}{3}\pi \operatorname{r}_{-}^{3}+\frac{13}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}+{{\operatorname{r}}_{-}} \right)}^{3}}}\]

    D) \[\frac{\frac{4}{3}\pi \operatorname{r}_{-}^{3}+\frac{13}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}+{{\operatorname{r}}_{-}} \right)}^{3}}}\]

    Correct Answer: A

    Solution :

    Effective number of remaining \[{{\operatorname{Cl}}^{-}}\]
    \[=4-\left[ 2\times \frac{1}{8}+\frac{1}{2} \right]=\frac{13}{4}\]
    Effective number of \[N{{a}^{+}}=4\]
    Packing Fraction \[=\frac{Occupied\,vol}{Total\,vol}\]
    \[=\frac{\frac{13}{4}\times \frac{4}{3}\pi \operatorname{r}_{-}^{3}+4\times \frac{4}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}-{{\operatorname{r}}_{-}} \right)}^{3}}}\]
    \[=\frac{\frac{13}{3}\pi \operatorname{r}_{-}^{3}+\frac{16}{3}\pi \operatorname{r}_{+}^{3}}{8{{\left( {{\operatorname{r}}_{+}}-{{\operatorname{r}}_{-}} \right)}^{3}}}\]


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