Chemistry • Year 12 • Module 6 • Lesson 6
Strong/Weak Mastery — Consolidation
Recall and organise the core vocabulary and classification rules for strong/weak acids and bases before moving to applied reasoning.
1. Term–definition match
The twelve definitions below are shuffled. In the right-hand column write the matching term from this list: strong acid, weak acid, strong base, weak base, degree of ionisation, Ka, Kw, conjugate acid, conjugate base, salt hydrolysis, concentration, pH. 12 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The negative logarithm (base 10) of the hydrogen ion concentration; pH = −log[H⁺]. | |
| 1.2 | An acid that ionises completely in aqueous solution; [H⁺] equals the molar concentration of the acid. | |
| 1.3 | An acid that only partially ionises in water; an equilibrium exists between intact molecules and their ions. | |
| 1.4 | A base that fully dissociates in aqueous solution; [OH⁻] equals the molar concentration of the base. | |
| 1.5 | A base that only partially accepts protons from water; most of the dissolved molecules remain intact. | |
| 1.6 | The fraction of dissolved acid (or base) molecules that have ionised, expressed as a percentage. | |
| 1.7 | The acid dissociation equilibrium constant; a measure of the intrinsic proton-donating tendency of an acid at a given temperature. | |
| 1.8 | The ion product of water; equal to [H⁺][OH⁻] = 1.0 × 10⁻¹&sup4; at 25°C. | |
| 1.9 | The species formed when a Brønsted-Lowry acid donates a proton; it can accept a proton in the reverse reaction. | |
| 1.10 | The species formed when a Brønsted-Lowry base accepts a proton; it can donate a proton in the reverse reaction. | |
| 1.11 | The reaction of an ion from a dissolved salt with water, shifting the pH away from 7; occurs when the parent acid or base is weak. | |
| 1.12 | The total amount of dissolved solute per litre of solution, expressed in mol/L; independent of the degree of ionisation. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, rewrite it correctly on the line below. 12 marks (1 T/F + 1 correction each)
2.1 A dilute solution of HCl is a weak acid because its pH is close to 7. T / F
2.2 Increasing the concentration of acetic acid (CH₃COOH) increases its Ka. T / F
2.3 Ca(OH)₂ is classified as a strong base because the dissolved fraction fully dissociates. T / F
2.4 A concentrated weak acid always has a higher pH than a dilute strong acid. T / F
2.5 The ionic equation for the ionisation of HNO₃ in water should use an equilibrium arrow (↔). T / F
2.6 The conjugate of a strong acid acts as a neutral spectator ion in solution and does not react with water. T / F
3. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
3.1 What is the difference between the terms strength and concentration when describing an acid?
3.2 Why is Ka described as an intrinsic property of an acid molecule?
3.3 What is the function of the salt hydrolysis rule in predicting the pH of a salt solution?
3.4 Why must the arrow used in the ionic equation for a strong acid be different from that used for a weak acid?
3.5 A student says “HF is dangerous so it must be a strong acid.” Why is this reasoning incorrect?
4. Fill in the blanks
Complete the passage below using the word bank. Each word is used once. 8 marks
Word bank: degree of ionisation • Ka • concentration • equilibrium • complete • partial • temperature • neutral
Whether an acid is classified as strong or weak depends on its __________ (1), not on how much acid is dissolved per litre. A strong acid undergoes __________ (2) ionisation in water; the forward reaction is so favoured that the reverse reaction is negligible. By contrast, a weak acid only partially ionises and an __________ (3) is established between the intact molecules and their ions. The __________ (4) quantifies this: for a strong acid it is effectively 100%, while for a weak acid at typical HSC concentrations it is usually less than 10%. The acid dissociation constant __________ (5) is a measure of the intrinsic strength and changes only with __________ (6), not with __________ (7). When a salt formed from a strong acid and a strong base dissolves in water, both ions are __________ (8) spectators and the resulting solution has pH 7.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are related. Each arrow must carry a linking phrase (e.g. “determines”, “is independent of”, “affects”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: acid strength (Ka) • concentration (c) • [H⁺] • degree of ionisation • pH • temperature.
6. Predict salt solution pH
For each salt, identify the parent acid and base, classify each as strong or weak, then predict whether the salt solution is acidic, basic, or neutral. 12 marks (2 each)
| Salt | Parent acid (strong/weak?) | Parent base (strong/weak?) | Solution pH prediction |
|---|---|---|---|
| NaCl | |||
| NH₄Cl | |||
| CH₃COONa | |||
| Na₂CO₃ | |||
| NH₄NO₃ | |||
| KNO₃ |
Q1 — Term–definition match
1.1 pH • 1.2 strong acid • 1.3 weak acid • 1.4 strong base • 1.5 weak base • 1.6 degree of ionisation • 1.7 Ka • 1.8 Kw • 1.9 conjugate base • 1.10 conjugate acid • 1.11 salt hydrolysis • 1.12 concentration.
Q2 — True or false with correction
2.1 False. HCl is a strong acid at any concentration; a high pH in a dilute solution is caused by low concentration, not partial ionisation. Correct: “A dilute solution of HCl is a dilute strong acid.”
2.2 False. Ka is an intrinsic property of the molecule; it changes only with temperature, not concentration. Increasing concentration does not change Ka.
2.3 True. Ca(OH)₂ is sparingly soluble, but every formula unit that does dissolve fully dissociates. The degree of ionisation of the dissolved fraction is 100% — it is a strong base.
2.4 False. A concentrated weak acid can have a lower pH than a dilute strong acid (e.g. 10 mol/L CH₃COOH has pH ≈ 1.87 vs 0.001 mol/L HCl at pH 3.0). The relationship between concentration, strength, and pH is not simple.
2.5 False. HNO₃ is a strong acid; it ionises completely, so a single forward arrow (→) is correct. The equilibrium arrow (↔) is used for weak acids.
2.6 True. The conjugate of a strong acid (e.g. Cl⁻, NO₃⁻) has essentially no tendency to accept a proton from water; it is a neutral spectator.
Q3 — Function recall
3.1 Strength (Ka) describes the fraction of molecules that ionise — an intrinsic property of the acid independent of how much is dissolved. Concentration (c, in mol/L) describes the total amount of acid dissolved per litre, regardless of how much has ionised. They are independent variables.
3.2 Ka is intrinsic because it reflects the bond energies and electronic structure of the acid molecule itself. It is the same whether you have 0.001 mol/L or 10 mol/L of the acid in solution; only temperature changes Ka.
3.3 The salt hydrolysis rule allows prediction of whether a salt solution will be acidic, basic, or neutral by tracing each ion back to its parent acid or base: conjugate of strong acid/base → neutral spectator; conjugate of weak acid → basic ion; conjugate of weak base → acidic ion.
3.4 A single forward arrow (→) communicates complete ionisation — the forward reaction is so strongly favoured that the reverse is negligible. An equilibrium arrow (↔) communicates partial ionisation — a significant amount of the intact molecule remains at equilibrium. Using the wrong arrow misrepresents the chemistry and costs marks.
3.5 Danger is determined by concentration, reactivity, and tissue toxicity — not by Ka. HF (Ka = 6.8 × 10⁻&sup4;) is a weak acid because it only partially ionises, but it is extremely dangerous because F⁻ ions penetrate tissue and cause systemic poisoning. Strength classification (Ka) says nothing about safety.
Q4 — Cloze answers (in order)
(1) degree of ionisation • (2) complete • (3) equilibrium • (4) degree of ionisation • (5) Ka • (6) temperature • (7) concentration • (8) neutral.
Q5 — Sample concept map
Acceptable arrows include: acid strength (Ka) → determines → degree of ionisation; degree of ionisation → determines fraction that contribute to → [H⁺]; concentration (c) → also determines total moles available to contribute to → [H⁺]; [H⁺] → determines → pH; temperature → is the only factor that changes → Ka; temperature → also affects → [H⁺] (via Kw). Award 1 mark per correctly labelled, directionally correct arrow, up to 6 marks.
Q6 — Salt solution pH predictions
NaCl: HCl (strong) + NaOH (strong) → both ions neutral spectators → neutral (pH 7).
NH₄Cl: HCl (strong) + NH₃ (weak) → NH₄⁺ is acidic ion (donates H⁺ to water) → acidic (pH < 7).
CH₃COONa: CH₃COOH (weak) + NaOH (strong) → CH₃COO⁻ is basic ion (accepts H⁺ from water) → basic (pH > 7).
Na₂CO₃: H₂CO₃ (weak) + NaOH (strong) → CO₃²– is strongly basic ion → basic (pH > 7).
NH₄NO₃: HNO₃ (strong) + NH₃ (weak) → NH₄⁺ is acidic ion; NO₃⁻ is neutral spectator → acidic (pH < 7).
KNO₃: HNO₃ (strong) + KOH (strong) → both ions neutral spectators → neutral (pH 7).