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NEET AIPMT SOLVED PAPER SCREENING 2007

  • question_answer
                    A weak acid, HA, has a \[{{K}_{a}}\] of \[1.00\times {{10}^{-5}}\]. If 0.100 mole of this acid is dissolved in one litre of water, the percentage of acid dissociated at equilibrium is closest to:                                                      

    A)                 99.0%     

    B)                            1.00%

    C)                 99.9%            

    D)                 0.100%

    Correct Answer: B

    Solution :

                              \[HA\rightleftharpoons {{H}^{+}}+{{A}^{-}}\]                 \[{{K}_{a}}=\frac{[{{H}^{+}}]\,[{{A}^{-}}]}{[HA]}=\frac{{{[{{H}^{+}}]}^{2}}}{[HA]}\]                 \[[{{H}^{+}}]=\sqrt{{{K}_{a}}\,[HA]}=\sqrt{1\times {{10}^{-5}}\times 0.1}\]                 \[=\sqrt{1\times {{10}^{-6}}}=1\times {{10}^{-3}}\]                 \[\alpha =\frac{actual\,ionization}{molar\,concentration}=\frac{{{10}^{-3}}}{0.1}={{10}^{2}}\]                 % of acid dissociated = \[{{10}^{-2}}\times 100\]                 = 1%


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