The titration curve of weak acid vs. strong base with phenolphthalein as indictor is shown below. The Kphenolphthalein = 4 × 10-10 . Given: log2 = 0.3 The number of following statements 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 pH = 8.4 C. It is a weak organic base D. It is colourless in acidic medium.
To determine the number of correct statements regarding phenolphthalein, we analyze each given statement:
Statement A: It can be used as an indicator for the titration of weak acid with weak base. This statement is incorrect because phenolphthalein is not suitable for titrations involving both weak acids and weak bases as the pH change is not sharp enough, making end point detection difficult.
Statement B: It begins to change colour at pH = 8.4. Given Kphenolphthalein = 4 × 10-10, the pKa is calculated as: \( \text{pK}_\text{a} = -\log(4 \times 10^{-10}) = 9.4 \). Phenolphthalein typically changes color around pH 8.2 to 10. Therefore, this statement is correct.
Statement C: It is a weak organic base. Phenolphthalein is actually a weak acid, not a base. This statement is incorrect.
Statement D: It is colourless in acidic medium. Phenolphthalein is colorless below pH 8.2 and thus is indeed colorless in acidic solutions. This statement is correct.
Counting the correct statements (B and D), the number of correct statements is 2.
This computed value matches the expected range of 2,2, confirming its validity.
