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KVPY Sample Paper KVPY Stream-SX Model Paper-9

  • question_answer
    \[{{H}_{2}}A\] is a weak triprotic acid \[({{K}_{{{a}_{1}}}}={{10}^{-\,5}},{{K}_{{{a}_{2}}}}={{10}^{-\,9}},{{K}_{{{a}_{3}}}}={{10}^{-\,13}})\]; What is the value of \[pX\]of 0.1 M \[{{H}_{3}}A\](aq.) solution ? where \[pX=-\,\log X\] and \[X=\frac{[{{A}^{3-}}]}{[H{{A}^{2-}}]}\]

    A) 7

    B) 8   

    C) 9

    D) 10

    Correct Answer: D

    Solution :

    \[[{{H}^{+}}]=\sqrt{{{K}_{{{a}_{1}}}}\times C}=\sqrt{{{10}^{-5}}\times 0.1}={{10}^{-3}}\] \[{{K}_{{{a}_{3}}}}=\frac{[{{H}^{+}}][{{A}^{3-}}]}{[H{{A}^{-2}}]}={{10}^{-13}}\] \[\Rightarrow \frac{[{{A}^{-3}}]}{[H{{A}^{-2}}]}={{10}^{-10}}\] \[pX=10\]


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