Determine the “pKa” of phenol if its acid dissociation constant is “ $K_{a} = 1.3 \times 1 0^{- 10}$ ”.

easy 1 mark Class 12 · Chemistry · Alcohols Phenols and Ethers

Answer

9.89

The correct answer is 9.89 — Thus the “pKa” of phenol is “ $9.89$ ”.

Step-by-step solution

Write the expression for “pKa”: $d i s pl a y s t y l e p K_{a} = - l o g_{10} K_{a}$ .
Substitute the given $K_{a}$ : $p K_{a} = - l o g_{10} (1.3 \times 1 0^{- 10})$ .
Calculate the logarithm: $l o g_{10} (1.3 \times 1 0^{- 10}) \approx - 9.889$ .
Take the negative: $p K_{a} a pp r o x 9.89$ .
Round to two decimal places, as required.
Thus the “pKa” of phenol is “ $9.89$ ”.

Common mistake: Forgetting to take the negative logarithm or using the wrong base

Concept tested: Acid dissociation constant of phenol

Published 10 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked