The titration of a weak base versus weak acid cannot be carried out by using acid-base indicator because

The titration of a weak base versus weak acid cannot be carried out by using acid-base indicator because
  1. there is no variation in \(\mathrm{pH}\) of the solution
  2. the variation in \(\mathrm{pH}\) is gradual with no steep change in pH near the equivalence point.
  3. the change in pH near the equivalence point does not encompasses an interval equal to the \(\mathrm{pH}\) transition range of the indicator
  4. no indicator can be found which changes colour at pH of equivalence point.

Solution

During the titration, a buffer solution of weak acid and salt of its cojugate base is formed. Its \(\mathrm{pH}\) is given by
\(\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}^{\circ}+\log ([\mathrm{salt}] /[\text { acid }])\)
From the given date, we find
\(4.14=\mathrm{p} K_{\mathrm{a}}^{\circ}+\log [10 /(50-10)]\)
This gives \(\mathrm{p} K_{\mathrm{a}}^{\circ}=4.14-\log (1 / 4)=4.14+0.60=4.74\)
After the addition of \(40 \mathrm{~mL}\) of \(\mathrm{NaOH}\) solution, we will have
\(\mathrm{pH}=4.74+\log [40 /(50-40)]=4.74+0.60=5.34\) ^

Asked in: JEE-TOPICTESTS-CHEMISTRY

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