The titration curve of weak acid vs. strong base with phenolphthalein as indictor) is shown below. The Kₚₕₑₙₒₗₚₕₜₕₐₗₑᵢₙ=4 × 10⁻¹⁰.
Given: log 2=0.3
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 pH=8.4
C. It is a weak organic base
D. It is colourless in acidic medium

Solution

Chemistry ma’am · 2 steps

Find pK_(In) first; the colour change spans one pH unit on either side of it.

1. Find pKIn. Phenolphthalein is a weak acid, HIn, with K_(In) = 4 × 10⁻¹⁰. Using log 4 = 2 log 2 = 0.6, you get pK_(In) = 10 - 0.6 = 9.4.

   pH range = pK_(In) ± 1 = 8.4 to 10.4

   Statement	Check	Verdict
   A. Works for weak acid vs weak base	There is no sharp pH jump at the equivalence point, so no indicator gives a clear end point	Wrong
   B. Begins to change colour at pH 8.4	The lower end of the range is 9.4 - 1 = 8.4	Correct
   C. Weak organic base	It is HIn, a weak organic acid	Wrong
   D. Colourless in acidic medium	In acid, the unionised HIn form dominates, and it is colourless. Pink appears only in basic medium	Correct

2. So. Only B and D are correct, so the answer is 2.

The common slip: 3. Counting C as correct gives 3. Phenolphthalein is a weak organic acid, HIn, not a base.

The idea to keep. An indicator changes colour over pK_(In) ± 1. Choose it so this range falls inside the steep jump of the titration curve. Weak acid vs weak base has no such jump.

Answer from the NTA final answer key. Solution by PrepWiser.