JEE MAIN - Chemistry (2023 - 8th April Morning Shift - No. 18)
The titration curve of weak acid vs. strong base with phenolphthalein as indictor) is shown below. The $$\mathrm{K}_{\text {phenolphthalein }}=4 \times 10^{-10}$$.
Given: $$\log 2=0.3$$
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The number of following statement/s which is/are correct about phenolphthalein is ___________
A. It can be used as an indicator for the titration of weak acid with weak base.
B. It begins to change colour at $$\mathrm{pH}=8.4$$
C. It is a weak organic base
D. It is colourless in acidic medium
Explanation
A. This statement is incorrect. Phenolphthalein cannot be used as an indicator for the titration of a weak acid with a weak base because the pH at the equivalence point is not within the color change range of phenolphthalein (8.2-10.0).
B. This statement is correct. Phenolphthalein begins to change color at a pH of around 8.2, and the transition completes around pH 10.0. It transitions from colorless to pink as the pH increases past this point.
C. This statement is incorrect. Phenolphthalein is not a base; it is a pH indicator, which can donate protons (like an acid) but not accept them (like a base).
D. This statement is correct. Phenolphthalein is indeed colorless in acidic medium (pH less than 7), and it turns pink in a basic solution (pH greater than 7).
So, the number of correct statements is 2.
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