2. Sample Papers
  3. KVPY

KVPY Sample Paper KVPY Stream-SX Model Paper-27

  • question_answer
    An acid-base indicator which is a weak acid has a \[p{{K}_{a}}\] \[\text{value}=5.45.\]At what concentration ratio of sodium acetate to acetic acid would the indicator show a colour half-way between those of its acid and conjugate base forms? \[p{{K}_{a}}\] of acetic \[\text{acid}=4.75.\]\[[\text{log }2=0.3]\]

    A) 4 : 1

    B) 7 : 1

    C) 5 : 1

    D) 2 : 1

    Correct Answer: C

    Solution :

    At half-way between two colours \[[H\,\,In]=[I{{n}^{-}}]\]
    Now      \[pKIn=pH+\log \frac{[H\,\,In]}{[I{{n}^{-}}]}=pH\]
    Now      pH of buffer = 5.45 when
                \[5.45=4.75+\log \frac{[\text{salt }\!\!]\!\!\text{ }}{[\text{acid }\!\!]\!\!\text{ }}\]
    So        \[\log \frac{[\text{salt }\!\!]\!\!\text{ }}{[\text{acid }\!\!]\!\!\text{ }}=0.70\]
                \[\frac{[\text{salt }\!\!]\!\!\text{ }}{[\text{acid }\!\!]\!\!\text{ }}=5:1\]


You need to login to perform this action.
You will be redirected in 3 sec spinner