Question

(A) In general phenolphthalein is used as an indicator for the titration of a weak acid (\[C{{H}_{3}}COOH\]) and strong base (\[NaOH\]).
(R) At the equivalence point solution is basic.
(A)- Both (R) and (A) are true and the reason is the correct explanation of assertion.
(B)- Both (R) and (A) are true but the reason is not the correct explanation of assertion.
(C) – Assertion (A) is true but reason (R) is false
(D)- Assertion (A) is false but reason (R) is true

Answer 1

Hint: At the equivalence point, the solution turns from colourless to pale pink colour.
The equivalence point is that point of the acid-base titration when the amount of the titrant added is sufficient to neutralize the analyte solution.
Phenolphthalein is a commonly used acid-base indicator and it has no colour in acidic conditions.

Complete step by step solution:
- In general, phenolphthalein is used as an indicator for the titration of a weak acid (\[C{{H}_{3}}COOH\]) and strong base (\[NaOH\]). This is because phenolphthalein changes its colour from colourless to pale pink at around a \[pH\]of 8 to 10 and the equivalence point for the titration of the weak acid and strong base lies around \[pH\]7 and upwards.
- Other indicators like methyl orange change its colour at around a \[pH\]of 4. So, there is a considerable difference between the \[pH\]of the colour change of the indicator and the equivalence point for the titration of the weak acid and strong base.
- So, phenolphthalein is the best indicator in this case.
The graph showing the equivalence point for the titration of the weak acid and strong base is as follows:




So, option B is the correct option.

Note: The reason behind the equivalence point for the titration of the weak acid and strong base lies around \[pH\]7 and upwards is due to the conjugate base of the weak acid. So, there will be a greater concentration of \[O{{H}^{-}}\]ions in the solution.