Calculate the pH of a 0.010 M solution of acetic acid (CH₃COOH) at 25 °C. Ka = 1.8×10⁻⁵.

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

Answer

3.37

The correct answer is 3.37 — Conclusion: The pH of the solution is 3.37.

Step-by-step solution

Write the dissociation of acetic acid: $ce C H 3 C O O H <=> H + + C H 3 C O O -$ .
Set $x = [ce H +] = [ce C H 3 C O O -]$ and $[C H 3 C O O H] = 0.010 - x$ .
Write the Ka expression: $K a = \frac{x ^{2}}{0.010 - x}$ .
Since $x$ will be small, approximate $0.010 - x a pp r o x 0.010$ .
Solve $x^{2} = K a \times 0.010 \Rightarrow x = 1.8 \times 1 0^{- 5} \times 0.010 = 4.24 \times 1 0^{- 4}$ .
Compute pH: $p H = - l o g_{10} (4.24 \times 1 0^{- 4}) = 3.37$ .
Conclusion: The pH of the solution is 3.37.

Common mistake: Ignoring the approximation

0.010 - x a pp r o x 0.010

or misusing the Ka expression

Concept tested: Acid dissociation and pH calculation

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