Methanoic acid (HCOOH) is a weak acid. Sulfuric acid (H₂SO₄) is a strong acid. Both acids are prepared as solutions of equal concentration. Explain why the methanoic acid solution has a higher pH than the sulfuric acid solution of equal concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methanoic acid is found naturally in ant venom. Sulfuric acid is a strong acid used widely in industry. Equal concentrations of the two acids are prepared in separate beakers.
Model answer (5 marks)
Sulphuric acid is a strong acid and is completely dissociated in solution:
H₂SO₄ → 2H⁺ + SO₄²⁻
Thus a 0.1 M solution gives 0.2 M H⁺.
Methanoic acid is a weak acid and only partially dissociates:
HCOOH ⇌ H⁺ + HCOO⁻
At 0.1 M the equilibrium lies far to the left, so the H⁺ concentration is only about 10⁻⁴ M.
Because the H⁺ concentration in the sulphuric acid solution is much higher, its pH (pH = –log[H⁺]) is lower. Therefore the methanoic acid solution has a higher pH than the sulphuric acid solution of equal concentration.
H₂SO₄ → 2H⁺ + SO₄²⁻
Thus a 0.1 M solution gives 0.2 M H⁺.
Methanoic acid is a weak acid and only partially dissociates:
HCOOH ⇌ H⁺ + HCOO⁻
At 0.1 M the equilibrium lies far to the left, so the H⁺ concentration is only about 10⁻⁴ M.
Because the H⁺ concentration in the sulphuric acid solution is much higher, its pH (pH = –log[H⁺]) is lower. Therefore the methanoic acid solution has a higher pH than the sulphuric acid solution of equal concentration.
Examiner tips
- Show the dissociation equations for both acids
- State that the strong acid is fully dissociated, the weak acid is only partially
- Link higher H⁺ concentration to lower pH
- Use the pH formula to justify the conclusion
Common mistakes
- Confusing the concentration of the acid with the concentration of H⁺
- Forgetting that the weak acid dissociation is an equilibrium
- Using the wrong sign in the pH calculation
Mark scheme (5 marks)
- Sulfuric acid is fully/completely dissociated (in solution) / ionised
- Methanoic acid is partially/only slightly dissociated (in solution) / ionised
- Therefore the sulfuric acid solution contains a greater concentration of H⁺ (hydrogen/hydronium) ions than the methanoic acid solution (at equal concentrations)
- pH is a measure of hydrogen ion concentration / higher H⁺ concentration gives a lower pH
- The reversible nature of weak acid dissociation means equilibrium lies to the left, so most molecules remain undissociated
Key terms in this question
weak acid · strong acid · pH
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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