6.1.6Additional lessons
Strong and Weak Acids — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 121 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Strength
Strength describes how far an acid or base ionises, whereas concentration describes how much is dissolved.
Strong acid
- Strong acid A strong acid dissociates almost completely in water, for example HCl(aq) → H⁺(aq) + Cl⁻(aq).
Weak acid
- Weak acid A weak acid dissociates only partially, for example CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq), with mostly undissociated molecules.
Strong base
- Strong base Strong bases such as NaOH and KOH are fully ionised in solution and release OH⁻ ions.
Weak base
- Weak base A weak base such as NH₃ reacts only slightly with water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Part 2 of 4Strength versus Concentration
Strength versus Concentration
Strength
- Strength This is a property of the substance, such as HCl always being strong and ethanoic acid always weak.
Concentration
- Concentration This is the amount per dm³;
- a concentrated solution contains a lot of solute and a dilute one only a little.
Possible combinations
- Possible combinations A dilute strong acid and a concentrated weak acid are both possible, so the two terms must not be confused.
Examples
- Examples HCl, HNO₃ and H₂SO₄ (first ionisation) are strong; CH₃COOH and H₂CO₃ are weak.
| Test at equal concentration | Strong acid | Weak acid |
|---|---|---|
| pH | Lower | Higher |
| Electrical conductivity | Higher | Lower |
| Rate with magnesium | Faster | Slower |
| Moles of H₂ from excess Mg | Same | Same |
Part 3 of 4Worked Example
Question Calculate the pH of 0.0050 mol dm⁻³ HCl and of 0.0050 mol dm⁻³ NaOH at 298 K.
- HCl is strong[H⁺] = 0.0050 mol dm⁻³, so pH = −log₁₀(0.0050) = 2.30.
- NaOH is strong[OH⁻] = 0.0050 mol dm⁻³, so [H⁺] = 1.0 × 10⁻¹⁴ / 0.0050 = 2.0 × 10⁻¹² mol dm⁻³.
- pH = −log₁₀(2.0 × 10⁻¹²) = 11.70.
Answer HCl: pH 2.30; NaOH: pH 11.70
Part 4 of 4Exam Checklist & Tips
6.1.6 – Key Points for the Exam
- Strong acid An acid that is almost completely dissociated in aqueous solution.
- Weak acid An acid that is only partially dissociated in aqueous solution.
- Dissociation The splitting of a substance into ions in solution.
- Strong base A base that is fully ionised in solution, such as NaOH.
Common Mistakes
- Using strong to mean concentrated and weak to mean dilute.
- Writing a single arrow for the dissociation of a weak acid.
- Thinking a weak acid produces less hydrogen gas than a strong acid of equal moles.
- Forgetting that a strong acid has a weak conjugate base.
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