Question

Phenolphthalein is a:
(A) Strong acid
(B) Weak acid
(C) (A) and (B) both
(D) None of the above

Answer 1

Hint: Phenolphthalein is an organic compound that is used as a laboratory agent and as a pH indicator. It is used as an indicator in acid-base titrations.

Complete step by step solution:
Phenolphthalein is a chemical compound having the chemical formula ${C}_{20}{H}_{14}{O}_{4}$ and is often written as PHP in short notation. It belongs to the phthalein family which is mostly used as an acid-base indicator. Because it indicates the pH of the solution, it is colourless below 8.5 pH and is able to attain a pink to deep red colour above 9.0 pH.
Phenolphthalein is a weak acid and it dissociates in water to give pink anions.
Let us see the reaction given below:

Under the acidic conditions, the equilibrium of the reaction shifts to the left and the concentration of the anions is far too low to observe the pink colour. However, when the reactions are conducted under alkaline conditions, the equilibrium shifts to the right and the concentration of the anions increases or becomes sufficient to observe the pink colour.
Hence, we can say that phenolphthalein is a weak acid.

Therefore, (B) is the correct answer.

Note: Phenolphthalein is water-soluble and can also dissolve in alcohols and this property is used for experiments. It is used to indicate the completion of an acid-base reaction. It is also used in sensors, semiconductors and in detecting viable cells. It is also used as a method to prevent drug misuse.