Question

The titration curve of weak acid vs. strong base with phenolphthalein as indictor is shown below. The K_{phenolphthalein} = 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.

Answer 1

Correct Answer: 2

The p$K_n$ for phenolphthalein is calculated as: \[ \text{p}K_n = -\log (4 \times 10^{-10}) = 9.4 \] The indicator range is given by: \[ \text{p}K_n \pm 1 \implies 8.4 \, \text{to} \, 10.4 \] \begin{itemize} \item Statement (B) is correct since phenolphthalein changes color around pH = 8.4. \item Statement (D) is correct as phenolphthalein is colorless in acidic medium due to its unionized form. \end{itemize}