Chemistry • Year 12 • Module 6 • Lesson 5
Strong vs Weak Acids & Bases: The Critical Distinction
Apply the strength–concentration distinction to real conductivity data, sequential reasoning chains, and an Australian industrial context.
1. Interpret conductivity and pH data at equal concentration
A student measured the electrical conductivity and pH of six solutions, each prepared at 0.10 mol/L, at 25°C. The data are shown below. 8 marks
| Solution | Concentration (mol/L) | Conductivity (mS/cm) | pH | Strong or weak? |
|---|---|---|---|---|
| HCl | 0.10 | 42.1 | 1.0 | to be completed |
| HNO₃ | 0.10 | 40.9 | 1.0 | to be completed |
| CH₃COOH | 0.10 | 0.53 | 2.9 | to be completed |
| HF | 0.10 | 6.1 | 2.1 | to be completed |
| NaOH | 0.10 | 24.5 | 13.0 | to be completed |
| NH₃ | 0.10 | 0.47 | 11.1 | to be completed |
Data adapted from laboratory measurements at 25°C. Conductivity measured using a calibrated conductivity probe.
1.1 Complete the “Strong or weak?” column for all six solutions. Justify your classification of HF in one sentence. 2 marks
1.2 Describe the relationship between conductivity and acid/base strength shown in this data set. Use at least two pairs of values to support your answer. 3 marks
1.3 A student argues that because HF has a lower pH than NH₃, HF must be a stronger acid than NH₃ is a base. Evaluate this claim. 3 marks
2. Graph interpretation — conductivity vs concentration
The graph below shows how electrical conductivity changes with concentration for two solutions: 0.1 mol/L HCl (strong acid) and 0.1 mol/L CH₃COOH (weak acid), measured across a dilution series from 0.001 to 0.10 mol/L. 7 marks
Figure 2.1. Conductivity vs concentration for HCl and CH₃COOH at 25°C. Adapted from laboratory data; values at 0.001–0.10 mol/L.
2.1 Describe the shape of the HCl conductivity curve. What does this shape indicate about the relationship between concentration and the number of ions in solution for a strong acid? 2 marks
2.2 At 0.04 mol/L, HCl conductivity is approximately 24.5 mS/cm while CH₃COOH conductivity is approximately 0.18 mS/cm. Using lesson content, account for this difference. 2 marks
2.3 Predict the conductivity of a 0.10 mol/L HBr solution. Justify your prediction with reference to both the graph and lesson content. 3 marks
3. Cause-and-effect chain — HCl in Australian industrial water treatment
HCl is used by companies such as CSL Behring (Melbourne) and municipal water treatment plants across Australia to adjust pH in process streams. The chain below traces what happens when HCl is added to a neutral water stream. Complete the empty effect boxes. 5 marks
Cause: HCl(aq) is added to a neutral water stream (pH 7).
Effect 1: HCl undergoes ______________ ionisation (write the ionic equation with the correct arrow): _______________________________________________
Effect 2: As a result, [H⁺] in the stream ______________ and [OH⁻] ______________ because the ion product of water Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ is constant at 25°C.
Effect 3: The pH of the stream ______________ below 7 because pH = −log[H⁺] and [H⁺] has ______________.
Effect 4: The electrical conductivity of the stream ______________ significantly because a strong acid produces a ______________ number of ions per mole dissolved compared with a weak acid at the same concentration.
Overall outcome (so…): Write one sentence linking HCl’s classification as a ______________ acid to why a small added volume causes a large, predictable drop in pH — unlike a weak acid at the same concentration would.
4. Case study — acetic acid in Australian food manufacturing
5 marks
Stimulus. The Australian food industry uses acetic acid (CH₃COOH) as a food preservative (food acid 260) and pH control agent in products including vinegar (typically 5% acetic acid by mass, ~0.83 mol/L), pickled vegetables and sourdough. The acetic acid in vinegar is responsible for the sharp taste and acts as a preservative by lowering water activity and pH. A food technologist at a South Australian pickle manufacturer noted that 0.83 mol/L acetic acid has a measured pH of approximately 2.4 — yet the same volume of 0.83 mol/L HCl would have a pH of approximately 0.08. The technologist described acetic acid as “a weaker version of HCl that has partially decomposed.”
4.1 Identify the scientific error in the technologist’s description of acetic acid. What is the correct explanation for why the two solutions at the same concentration have different pH values? 3 marks
4.2 Write the correct ionic equation for acetic acid dissolving in water, including state symbols and the correct arrow. Explain what the arrow communicates. 2 marks
Q1.1 — Classification
HCl: strong. HNO₃: strong. CH₃COOH: weak. HF: weak (HF is not on the strong acid list; Ka = 6.8 × 10⁻⁴, indicating only partial ionisation). NaOH: strong. NH₃: weak.
Q1.2 — Conductivity and strength relationship
At the same concentration (0.10 mol/L), strong acids and bases have far higher conductivity than weak ones. For example, HCl (42.1 mS/cm) vs CH₃COOH (0.53 mS/cm); NaOH (24.5 mS/cm) vs NH₃ (0.47 mS/cm). The higher conductivity of strong acids/bases reflects complete dissociation producing a large number of ions per mole, whereas weak acids/bases produce far fewer ions because only a small fraction ionises.
1 mark for stating strong acids/bases have higher conductivity at same concentration; 1 mark for correctly quoting at least two pairs of values; 1 mark for linking conductivity to ion concentration and degree of dissociation.
Q1.3 — Evaluating the pH comparison claim
The claim is flawed. Acid and base strength cannot be compared using pH alone when different substances are involved, because pH measures [H⁺] (for acids) and [OH⁻] (for bases) on entirely different scales. HF’s pH of 2.1 measures how acidic it is; NH₃’s pH of 11.1 measures how basic it is. Both are weak electrolytes with low degrees of dissociation — as shown by their similarly low conductivities (6.1 and 0.47 mS/cm). Comparing an acid’s pH to a base’s pH to judge relative strength is meaningless.
1 mark for identifying the claim is flawed; 1 mark for explaining pH cannot compare acid and base strength; 1 mark for using conductivity data or degree of dissociation as the correct strength criterion.
Q2.1 — HCl curve shape
The HCl curve is approximately linear (straight line through the origin). This indicates that [H⁺] is directly proportional to concentration for a strong acid — because every HCl molecule ionises completely, doubling the concentration exactly doubles the number of ions (H⁺ and Cl⁻) in solution.
Q2.2 — Conductivity difference at 0.04 mol/L
HCl is a strong acid and ionises completely at 0.04 mol/L, so effectively all HCl molecules exist as H⁺ and Cl⁻ ions, producing a high ion concentration and high conductivity. CH₃COOH is a weak acid and only partially ionises (a small fraction establish equilibrium), so the vast majority of molecules remain as intact CH₃COOH, producing very few ions and very low conductivity.
Q2.3 — HBr prediction
HBr is a strong acid (on the strong acid list: HCl, H₂SO₄, HNO₃, HClO₄, HBr, HI). At 0.10 mol/L, HBr would ionise completely: HBr(aq) → H⁺(aq) + Br⁻(aq), producing [H⁺] = [Br⁻] = 0.10 mol/L. The conductivity would be very close to that of HCl at the same concentration (~40–43 mS/cm), consistent with the linear trend shown by HCl on the graph. The slight difference from HCl reflects the different molar conductivity of Br⁻ vs Cl⁻.
1 mark for identifying HBr as strong; 1 mark for predicting conductivity close to HCl (~40–43 mS/cm) with reference to graph; 1 mark for correctly writing the ionic equation with →.
Q3 — Cause-and-effect chain
Effect 1: HCl undergoes complete ionisation. Ionic equation: HCl(aq) → H⁺(aq) + Cl⁻(aq).
Effect 2: [H⁺] increases; [OH⁻] decreases.
Effect 3: pH decreases/falls below 7; [H⁺] has increased.
Effect 4: Conductivity increases significantly; HCl produces a large / greater number of ions per mole dissolved.
Overall outcome: Because HCl is a strong acid and ionises completely, every mole of HCl added releases exactly 1 mole of H⁺ ions, making pH change precise and predictable; a weak acid at the same concentration would release far fewer H⁺ ions and cause a smaller, harder-to-predict pH drop.
Q4.1 — Technologist’s error
The error is describing acetic acid as “a weaker version of HCl that has partially decomposed.” CH₃COOH has not decomposed — it is a chemically distinct compound with a different molecular structure and an intrinsically lower Ka (1.8 × 10⁻⁵ vs HCl which is essentially infinite). The pH difference at the same concentration reflects a difference in acid strength (degree of ionisation), not any change to the HCl molecule. CH₃COOH is a weak acid because only a small fraction of its molecules donate a proton to water at equilibrium; HCl is a strong acid because essentially every molecule donates its proton.
1 mark for identifying “partially decomposed” as wrong; 1 mark for explaining CH₃COOH and HCl are different compounds with different Ka; 1 mark for explicitly linking pH difference to degree of ionisation (strength).
Q4.2 — Ionic equation for acetic acid
CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
The equilibrium arrow (⇌) communicates that the ionisation is reversible and partial — both the forward (proton donation to water) and reverse (proton recapture by acetate) reactions occur simultaneously, establishing a dynamic equilibrium with most molecules remaining as intact CH₃COOH.