Given two or three unlabelled solutions of the same concentration, you explain their pH order and their electrical conductivity. Asked in 2021, 2023, 2024 and 2025. The examiners are testing one idea: degree of dissociation.
The two links you must make explicitly
pH → degree of dissociation → [H₃O⁺]. More dissociation means more H₃O⁺ means lower pH.
Electrical conductivity → degree of dissociation → [ions]. Conductivity depends on the total concentration of ions, not just H₃O⁺.
Schedules ask for supporting equations — one per solution. Write the dissociation equation for each, using → for strong and ⇌ for weak.
Same concentration, different behaviour
Solution
Dissociation
[ions]
Conductivity
pH
Strong acid (HCl)
Complete: HCl → H₃O⁺ + Cl⁻
High
High
Lowest
Weak acid (CH₃COOH)
Partial: CH₃COOH ⇌ H₃O⁺ + CH₃COO⁻
Low
Low
Higher than HCl
Strong base (NaOH)
Complete: NaOH → Na⁺ + OH⁻
High
High
Highest
Weak base (NH₃)
Partial: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Low
Low
Lower than NaOH
Salt (NaCl)
Fully ionic, no hydrolysis
High
High
≈ 7
✕
The trap: a salt conducts well but is neutral
A soluble salt like NaCl is fully dissociated, so it conducts as well as a strong acid — but its pH is about 7. Conductivity tells you about total ions; pH tells you about H₃O⁺ specifically. Using conductivity to argue about acidity is the single most common way students lose these marks. Use both measurements, separately. Worked example Two acids at the same concentration 0.1 mol L⁻¹ HCOOH and 0.1 mol L⁻¹ HCl. Explain which has the lower pH and which conducts better. HCl is a strong acid: HCl + H₂O → H₃O⁺ + Cl⁻, essentially complete. HCOOH is a weak acid: HCOOH + H₂O ⇌ H₃O⁺ + HCOO⁻, only partial. So HCl has a greater degree of dissociation → higher [H₃O⁺] → lower pH. It also produces a higher total [ions] → better electrical conductivity. ✓ HCl: lower pH and better conductivity, both because it dissociates completely while methanoic acid dissociates only partially. ✓
If they give you Ka values
A larger Ka (smaller pKa) means a greater degree of dissociation, so a lower pH at the same concentration. 2024 used exactly this to ask why the equivalence-point pH changes when HF is swapped for CH₃COOH. Quote the magnitude of Ka explicitly — the schedule asks for it by name.