Calculate the pH of a 0.020 M solution of formic acid (HCOOH) at 25 °C. Ka = 1.8×10⁻⁴.

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

Answer

2.72

The correct answer is 2.72 — Answer: 2.72.

Step-by-step solution

Write dissociation: $ce H C O O H <=> H + + H C O O -$ .
Set $x = [ce H +] = [ce H C O O -]$ and $[ce H C O O H] = 0.020 - x$ .
Ka expression: $K a = \frac{x ^{2}}{0.020 - x}$ .
Approximate $0.020 - x a pp r o x 0.020$ because $x$ is small.
Solve: $x^{2} = 1.8 \times 1 0^{- 4} \times 0.020 \Rightarrow x = 3.6 \times 1 0^{- 6} = 1.90 \times 1 0^{- 3}$ .
Compute pH: $p H = - l o g_{10} (1.90 \times 1 0^{- 3}) = 2.72$ .
Answer: 2.72.

Common mistake: Failing to square root or using wrong Ka

Concept tested: pH of weak acid solution

Published 11 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked