The pH of a 0.1 M CH3COOH solution is

• A. Less than 1
• B. Greater than 1, but less than 7
• C. Equal to 7
• D. Greater than 7 but less than 14

Answer: (B)

Weak acids don’t fully dissociate in solution, so the hydrogen ion concentration is smaller than the formal concentration. For acetic acid in 0.1 M solution, the equilibrium CH3COOH ⇌ H+ + CH3COO− gives Ka ≈ 1.8×10−5. With x = [H+], Ka = x^2/(0.1 − x). Since x is small compared to 0.1, x ≈ sqrt(Ka × 0.1) ≈ sqrt(1.8×10−6) ≈ 1.3×10−3 M. The pH is −log10(1.3×10−3) ≈ 2.9. So the solution is acidic but not extremely so, placing the pH between 1 and 7. This is why the correct characterization is that the pH is greater than 1 but less than 7.