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JEE Main & Advanced Chemistry Solutions / विलयन Question Bank Depression of freezing point of the solvent

  • question_answer
    A solution of urea \[\left( mol.\text{ }mass\text{ }56g\text{ }mo{{l}^{-1}} \right)\] boils at 100.18°C  at the atmospheric pressure. If \[{{K}_{f}}\] and \[{{K}_{b}}\]for water are 1.86 and \[0.512K\text{ }kg\text{ }mo{{l}^{-1}}\] respectively the above solution will freeze at      [CBSE PMT 2005]

    A) \[\,- 6.54{}^\circ C\]

    B) \[6.54{}^\circ C\]

    C) \[0.654{}^\circ C\]

    D) \[-0.654{}^\circ C\]

    Correct Answer: D

    Solution :

    \[\Delta {{T}_{b}}=0.18\]; \[\Delta {{T}_{b}}=m{{K}_{b}}\]                    \[\frac{0.18}{\Delta {{T}_{f}}}=\frac{m{{K}_{b}}}{m\,{{K}_{f}}}\]; \[\frac{0.18\times 1.86}{0.512}=\Delta {{T}_{f}}\] ; \[\Delta {{T}_{f}}=0.653\]                                 \[{{T}^{0}}-{{T}_{s}}=0.653\];\[{{T}^{0}}-{{T}_{s}}=0.653\]; \[{{T}_{s}}=0-{{0.653}^{o}}C\].


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