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JEE Main & Advanced Sample Paper JEE Main Sample Paper-32

  • question_answer
    A certain acid-base indicator is red in acid solution and blue in basic solution. At pH = 5.75% of the indicator is present in the solution in its blue form. Calculate the pH at which the indicator shows 90% red form? \[\left( Given\text{ }{{10}^{-4.523}}=3\times {{10}^{-5}} \right)\]

    A) 3.56       

    B)                                    5.47

    C) 2.5             

    D)                    7.4

    Correct Answer: A

    Solution :

    \[pH=p{{K}_{1}}\,+\log \,\frac{[{{\ln }^{-1}}]}{[H\ln ]}\]                 \[5=p{{K}_{1}}\,+\log \,\frac{75}{25}\]                 \[\Rightarrow \,\,\,\,\,p{{K}_{1}}=4.523\]                 \[\Rightarrow \,\,\,\,{{K}_{1}}=3\times {{10}^{-3}}\]                 \[pH=4.523\,+\log \,\frac{10}{90}\,=4.523\,-0.954\,=3.56\]


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