KVPY Sample Paper KVPY Stream-SX Model Paper-29

  • question_answer
    What is the freezing point of a solution containing 8.1 g HBr in 100 g water assuming the acid to be 90% ionised? \[({{K}_{f}}. for water = 1.86 K kg mo{{l}^{-1}}):\]

    A) 0.85 K

    B) -3.53K

    C) 0 K

    D) -0.38K

    Correct Answer: B

    Solution :

    Given mass of solute = 8.1 g
    Mass of solvent = 100 g
    For \[HBr\]
    \[\alpha =90%\text{ }=0.9\]
    \[i=-1+\alpha =1+0.9=1.9\]
    \[\Delta {{T}_{f}}={{K}_{f}}\times m\times i\]
    \[=1.86\times \frac{moles\,of\,solute}{mass\,of\,solvent\,in\,kg}\times 1.9\]
    \[=1.86\times \frac{8.1/81}{100/1000}\times 1.9\]
    \[=1.86\times 1\times 1.9=3.534K\]
    \[{{\operatorname{T}}_{f}}=T_{f}^{{}^\circ }-\Delta {{\operatorname{T}}_{f}}\]
    or \[{{\operatorname{T}}_{f}}=0-3.534K\]
    \[\therefore \,\,\,{{\operatorname{T}}_{f}}=-3.534K\]

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