Lesson eLearning

Calculate the pH of a 0.10 M solution of formic acid (HCOOH, Ka = 1.8×10⁻⁴) at 25 °C.

medium 2 marks Class 12 · Chemistry · Aldehydes Ketones and Carboxylic Acids
Answer
pH = 2.38

The correct answer is pH = 2.38 — Calculate pH: pH = –log[H⁺] = –log(4.24×10⁻³) = 2.38.

Step-by-step solution

  1. Write the dissociation expression: HCOOH ⇌ H⁺ + HCOO⁻, with Ka = [H⁺][HCOO⁻]/[HCOOH].
  2. Assume the degree of dissociation is small, let x = [H⁺] = [HCOO⁻] at equilibrium and [HCOOH] ≈ 0.10 M.
  3. Set up the equation: Ka = x² / (0.10 − x) ≈ x² / 0.10.
  4. Solve for x: x = √(Ka × 0.10) = √(1.8×10⁻⁴ × 0.10) = √(1.8×10⁻⁵) = 4.24×10⁻³ M.
  5. Calculate pH: pH = –log[H⁺] = –log(4.24×10⁻³) = 2.38.
Common mistake: Using the exact expression without approximating and solving a quadratic incorrectly.

Concept tested: pH of weak acid solution

Published · How this solution was prepared and checked

← All Aldehydes Ketones and Carboxylic Acids questions