Calculate the pH of a 0.025 M solution of acetic acid (CH3COOH) at 25 °C, given its acid dissociation constant Ka = 1.8×10⁻⁵.

Step-by-step solution

1. Step 1: Set up the equilibrium expression for acetic acid dissociation: [CH3COOH] ⇌ [CH3COO⁻] + [H⁺],\ Ka = \frac{[CH3COO⁻][H⁺]}{[CH3COOH]}.
2. Step 2: Let x = [H⁺] = [CH3COO⁻]. Since initial concentration is 0.025 M, [CH3COOH] ≈ 0.025 – x. For weak acids, x is small compared to 0.025, so approximate [CH3COOH] ≈ 0.025.
3. Step 3: Substitute into Ka expression: 1.8×10⁻⁵ = \frac{x²}{0.025}.
4. Step 4: Solve for x: x² = 1.8×10⁻⁵ × 0.025 = 4.5×10⁻⁷, \ x = √(4.5×10⁻⁷) = 6.708×10⁻⁴ M.
5. Step 5: Compute pH: pH = -log₁₀(6.708×10⁻⁴) = 3.173. Rounded to two decimal places: 3.17.

Concept tested: pH calculation for weak acids

Published 7 October 2026 · How this solution was prepared and checked