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NEET Sample Paper NEET Sample Test Paper-80

  • question_answer
    An acid-base indicator has a \[{{\operatorname{K}}_{a}}\,\,of\, 3 \times  1{{0}^{-5}}\]. The acid form of the indicator            is red and the basic form is blue. The change in \[{{H}^{+}}\] concentration \[\left( which is 9 \times \,\,{{10}^{-}}^{5} \right)\] in order to change the indicator from \[25\,%\] red to \[75\,%\] blue is

    A)  \[2\,\,\times \,\,{{10}^{-}}^{5}\]                    

    B)  \[8\,\,\times \,\,{{10}^{-}}^{5}\]

    C)  \[9\,\,\times \,\,{{10}^{-}}^{5}\]                    

    D)  \[1\,\,\times \,\,{{10}^{-}}^{5}\]

    Correct Answer: B

    Solution :

    \[\underset{Acidic\,\,red}{\mathop{HIn}}\,\,\,\xrightarrow{{}}\,\,{{H}^{+}}+\underset{Basic\,\,blue}{\mathop{I{{n}^{-}}}}\,\] \[{{K}_{a}}=\frac{[{{H}^{+}}][I{{n}^{-}}]}{[HIn]}\] When \[75\,%\] blue and \[25\,%\] red, then \[[{{H}^{+}}]=3\times {{10}^{-\,5}}\times \frac{25}{75}=1\times {{10}^{-\,5}}\] \[\therefore \,\,\,Change\,\,in\,\,[{{H}^{+}}]\,\,=\,\,9\times 10{{~}^{-\,5}}-1\times 1{{0}^{-}}^{8}\times 1{{0}^{-\,5}}\]


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