JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Question Bank Hydrogenion concentration pH scale and buffer solution

  • question_answer
    At 25°C, the dissociation constant of a base BOH is \[1.0\times {{10}^{-12}}\]. The concentration of Hydroxyl ions in 0.01 M aqueous solution of the base would be [CBSE PMT 2005]

    A)                 \[2.0\times {{10}^{-6}}mol\,{{L}^{-1}}\]

    B)                 \[1.0\times {{10}^{-5}}mol\ {{L}^{-1}}\]

    C)                 \[1.0\times {{10}^{-6}}mol\,{{L}^{-1}}\]

    D)                 \[1.0\times {{10}^{-7}}mol\ {{L}^{-1}}\]

    Correct Answer: D

    Solution :

                                   \[BOH\rightleftharpoons {{B}^{+}}+O{{H}^{-}}\] Initial              C                                        0                         0 At eq.          C ? Ca               Ca                       Ca                                        \[{{K}_{b}}=\frac{{{C}^{2}}{{\alpha }^{2}}}{C(1-\alpha )}=C{{\alpha }^{2}}\] assuming \[\alpha <<1\];\[1-\alpha \tilde{}1\]             \[{{10}^{-12}}={{10}^{-2}}\times {{\alpha }^{2}}\]; \[{{\alpha }^{2}}={{10}^{-10}}\]; \[\alpha ={{10}^{-5}}\]                                 \[[O{{H}^{-}}]=C\alpha =.01\times {{10}^{-5}}={{10}^{-7}}\]


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