Manipal Medical Manipal Medical Solved Paper-2014

  • question_answer
    1.0 g of a weak monobasic acid (HA) when dissolved in 150 mL water lowers the freezing point by 0.186° C. Also 1.0 g of the same acid required 125 mL of a 0.10N NaOH solution for complete neutralisation. Determine dissociation constant (\[{{k}_{a}}\]) of the weak acid. \[{{k}_{f}}\]of water is 1.86K kg \[mo{{l}^{-1}}\]

    A) \[4.16\times {{10}^{-3}}\]                               

    B) \[2.38\times {{10}^{-6}}\]

    C) \[6.43\times {{10}^{-3}}\]                               

    D) \[3.78\times {{10}^{-6}}\]

    Correct Answer: A

    Solution :

    From neutralization reaction, meq of acid \[=125\times 0.1=12.5\] Eq. wt. of acid \[=\frac{1000}{12.5}=80\] mol. wt. Molarity \[=\frac{1}{80}\times \frac{100}{15}=0.083\] \[-\Delta {{T}_{f}}=i{{k}_{f}}m=(1+\alpha )\times 1.86\times 0.083\] \[\alpha =0.2\] \[{{k}_{a}}=\frac{C{{\alpha }^{2}}}{1-\alpha }=\frac{0.093\times {{(0.2)}^{2}}}{0.8}=4.16\times {{10}^{-3}}\]


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