Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A:…
Redox Reactions · Class 11 · JEE Main Previous Year Question
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Phenolphthalein is a pH dependent indicator, remains colourless in acidic solution and gives pink colour in basic medium. Reason R: Phenolphthalein is a weak acid. It doesn't dissociate in basic medium.
In the light of the above statements, choose the most appropriate answer from the options given below:
- a
Both A and R are true and R is the correct explanation of A
- b
Both A and R are true but R is NOT the correct explanation of A.
- c✓
A is true but R is false
- d
A is false but R is true
A is true but R is false
Step 1 — Evaluate Assertion A:
Phenolphthalein:
- In acidic medium (pH < 8.3): exists as (colourless lactone form) ✓
- In basic medium (pH > 10): exists as (pink quinonoid form) ✓
- It is indeed pH-dependent
Assertion A is TRUE ✓
Step 2 — Evaluate Reason R:
Phenolphthalein IS a weak acid — this part is correct.
However, the statement "It doesn't dissociate in basic medium" is INCORRECT. In basic medium, phenolphthalein dissociates (loses protons) to form the coloured ion:
The pink colour in basic medium is precisely BECAUSE it dissociates in basic medium.
Reason R is FALSE ✗
Answer: Option (3) — A is true but R is false
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