What is the pH of a 0.020 M solution of propanoic acid (CH₃CH₂COOH) at 25 °C? Ka = 1.8×10⁻⁵.

easy 1 mark Class 12 · Chemistry · Aldehydes Ketones and Carboxylic Acids

Answer

3.22

The correct answer is 3.22 — Therefore the pH is 3.22.

Step-by-step solution

Let $x$ be the concentration of H⁺ produced by the acid.
Write the expression $K_{a} = \frac{x ^{2}}{0.020 - x}$ .
Because $x l l 0.020$ , approximate $0.020 - x a pp r o x 0.020$ .
Solve for $x$ : $x = K_{a} \times 0.020 = 1.8 \times 1 0^{- 5} \times 0.020 = 6.0 \times 1 0^{- 4},$ M.
Calculate the pH: $pH = - l o g_{10} (x) = - l o g_{10} (6.0 \times 1 0^{- 4}) = 3.22$ .
Therefore the pH is 3.22.

Common mistake: Treating the acid as a strong acid and adding the concentrations

Concept tested: pH of weak carboxylic acid

Published 14 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked