Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R:
Assertion A: pKₐ value of phenol is 10.0 while that of ethanol is 15.9 .
Reason R: Ethanol is stronger acid than phenol.
In the light of the above statements, choose the correct answer from the options given below:

Solution

Chemistry ma’am · 4 steps

Lower pKa means stronger acid, so these numbers already tell you phenol beats ethanol.

1. Read the pKa. pKₐ = -log Kₐ, so a smaller pKₐ means a stronger acid. Phenol (10.0) is about 10^(5.9), nearly a million times, more acidic than ethanol (15.9).
2. Why phenol wins. In phenoxide, C₆H₅O⁻, the negative charge spreads into the benzene ring by resonance. In ethoxide, C₂H₅O⁻, the charge stays on oxygen, and the +I effect of the ethyl group pushes even more electron density onto it.
3. Check each statement. Assertion A gives the correct values, so it is true. Reason R says ethanol is the stronger acid, which is the reverse of the truth, so it is false.
4. So. A is true but R is false, which is option (d).

The idea to keep. Always compare acids through the stability of their conjugate bases. Resonance spreading the negative charge beats a localised charge, and a smaller pKa means the stronger acid.

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