Question

Assertion: In general, phenolphthalein is used as an indicator for the titration of weak acid (HA) against strong base (NaOH). Reason: At equivalence point, the solution is basic.
A. Both assertion and reason are correct and reason is the right explanation of assertion.
B. Both assertion and reason are correct but reason is not the correct explanation of the assertion.
C. Assertion is correct but reason is not correct.
D. Assertion is not correct but reason is correct.
D. Both assertion and reason are not correct.

Answer 1

Titration is a method which is used for calculating the concentration of an unknown solution with the help of the concentration of a known solution. There is a special apparatus used to carry out titration. Phenolphthalein is an indicator used in titration for indicating basic solutions.

Complete step by step answer:
We know that titration is a process of calculating the concentration of an unknown solution with help of concentration of a known solution. The solutions are taken in the respective apparatus and then drop by drop the solution from the burette is collected in the titration flask and then an indicator is used to indicate the presence of the unknown solution.
Phenolphthalein is used as an indicator in the titration process to indicate the presence of a base. It changes its colour from colourless to pink in the presence of a basic solution. It has a pH from 8.2-10 for detection of base.
This type of titration is known as the acid-base titration. In the given question we have a weak acid (HA) against a strong base (NaOH) and after the titration for the solutions take place on approaching the neutralisation point, the weak acid forms a salt with the strong base.
Once the salt is obtained then it has to undergo the hydrolysis process. Thus, hydrolysis of the salt formed gives an alkaline solution which has a range of pH 8.2 -10 and thus, phenolphthalein is used as an indicator for the titration of weak acid (HA) against strong base (NaOH).
Hence, the correct option for the above question is (B) as the assertion has not been justified with the reason given.

So the answer is option B.

Note: There are various other indicators used in the process of titration to identify the unknown solution which include methyl red for a pH of 4.2-6.2, litmus test is the most common test for identifying acid and basic solutions with a pH of 5-8 and bromothymol blue for a rage of 6-7.