HSC Exam Practice

Sample response. The degree of ionisation is the fraction of dissolved acid molecules that have ionised to form H⁺(aq) and the conjugate base anion in solution, expressed as a percentage. It equals ([H⁺] / c(acid)) × 100%.

Marking notes. 1 mark for “fraction of molecules that have ionised” or equivalent; 1 mark for expressing it as a percentage or ratio relative to original concentration. Must not define it as “how strong the acid is”.

Sample response. The four strong bases are NaOH, KOH, Ca(OH)₂, and Ba(OH)₂. Ca(OH)₂ is classified as strong because every formula unit that dissolves in water fully dissociates into Ca²⁺(aq) and 2OH⁻(aq); the dissolved fraction is 100% dissociated. Its low solubility limits the total amount of OH⁻ produced per litre (concentration effect), but this does not affect the degree of ionisation of the dissolved fraction, which remains 100%. Strength refers to the degree of ionisation of the dissolved species, not to solubility.

Marking notes. 1 mark for all four strong bases listed correctly; 1 mark for stating that the dissolved fraction of Ca(OH)₂ fully dissociates (100%); 1 mark for explicitly distinguishing solubility from strength/degree of ionisation.

Sample response. (a) HCl(aq) → H⁺(aq) + Cl⁻(aq) [single forward arrow — strong acid, complete ionisation]. (b) CH₃COOH(aq) ⇆ H⁺(aq) + CH₃COO⁻(aq) [equilibrium arrow — weak acid, partial ionisation]. (c) NH₃(aq) + H₂O(l) ⇆ NH₄⁺(aq) + OH⁻(aq) [equilibrium arrow — weak base].

Marking notes. 1 mark per equation; each mark requires both the correct species AND the correct arrow type. An incorrect arrow type forfeits that mark.

Sample response. Dilute describes concentration (mol/L) — how much acid is dissolved per litre — whereas weak describes degree of ionisation (Ka) — the fraction of dissolved molecules that ionise. They are independent: any combination is possible. Examples: (1) dilute strong acid — 0.001 mol/L HCl (low c, 100% ionised); (2) concentrated strong acid — 12 mol/L HCl (high c, 100% ionised); (3) dilute weak acid — 0.001 mol/L CH₃COOH (low c, partially ionised); (4) concentrated weak acid — 10 mol/L CH₃COOH (high c, still only ~0.1% ionised).

Marking notes. 1 mark for correct definition of dilute (concentration/mol/L); 1 mark for correct definition of weak (degree of ionisation/Ka); 1 mark for two correct examples; 1 mark for two additional correct examples covering all four combinations.

Sample response. Three-step salt hydrolysis process: (1) identify the parent acid and base that formed the salt; (2) classify each as strong or weak; (3) apply the rule: conjugate of strong acid/base = neutral spectator (no hydrolysis); conjugate of weak acid = basic ion (accepts H⁺ from water, raises pH); conjugate of weak base = acidic ion (donates H⁺ to water, lowers pH). Application to NaHCO₃: (1) parent acid = H₂CO₃ (carbonic acid); parent base = NaOH. (2) NaOH is a strong base; H₂CO₃ is a weak acid. (3) Na⁺ is the conjugate of NaOH (strong) = neutral spectator. HCO₃⁻ is the conjugate of H₂CO₃ (weak acid) = basic ion: HCO₃⁻ + H₂O ⇆ H₂CO₃ + OH⁻. Therefore NaHCO₃ solution is basic (pH > 7, approximately 8.3).

Marking notes. 1 mark for all three steps stated; 1 mark for correctly identifying parents of NaHCO₃ and classifying them; 1 mark for applying the rule correctly to HCO₃⁻; 1 mark for predicting pH > 7 with the hydrolysis equation.

Sample response. Method 1 — Electrical conductivity: at the same 0.10 mol/L, HCl (strong, 100% ionised) produces ≈0.20 mol/L total ions and conductivity ≈39 mS/cm. CH₃COOH (weak, ~1.3% ionised) produces ≈0.0026 mol/L total ions and conductivity ≈0.5 mS/cm. A conductivity meter or light-bulb apparatus clearly distinguishes the two. Method 2 — Initial reaction rate with Mg ribbon (or Na₂CO₃ powder, or CaCO₃ chips): HCl reacts rapidly (large initial [H⁺] = 0.10 mol/L); CH₃COOH reacts slowly initially (small initial [H⁺] ≈0.0013 mol/L), even though both eventually consume the same mass of Mg. The rate of H₂ gas evolution can be measured as bubble count per second or with a gas syringe.

Marking notes. 2 marks per method: 1 for correctly identifying the method; 1 for explaining how the result differs between the two acids and linking the observation to degree of ionisation. Accept any two valid methods; both must be non-pH-based.

Part (a) — Classification (3 marks). P (HNO₃): strong acid. pH 1.3 at 0.050 mol/L; [H⁺] = 10⁻1.3 = 0.050 mol/L = c (100% ionised). Conductivity 19.2 mS/cm is proportionally high (consistent with full ionisation at 0.050 mol/L; HCl at 0.10 mol/L gives 39.1, so 0.050 mol/L HNO₃ giving ≈19 is expected for a strong acid). Q (HCN): very weak acid. pH 5.1 is barely below 7, indicating tiny [H⁺]; conductivity 0.009 is extremely low, consistent with almost no ionisation. R (HNO₂): weak acid. pH 2.3, conductivity 2.9 — ionised some but not completely (degree of ionisation much less than 100%). S (HCOOH): weak acid. pH 2.5, conductivity 1.8 — similar argument to R. 1 mark for P correctly identified as strong with data justification; 1 mark for Q as very weak with data; 1 mark for R and S both correctly identified as weak with data.

Part (b) — Degree of ionisation calculations (4 marks). R (HNO₂, pH 2.3): [H⁺] = 10⁻2.3 = 5.01 × 10⁻³ mol/L. Degree = (5.01 × 10⁻³ / 0.050) × 100% = 10.0%. S (HCOOH, pH 2.5): [H⁺] = 10⁻2.5 = 3.16 × 10⁻³ mol/L. Degree = (3.16 × 10⁻³ / 0.050) × 100% = 6.3%. 1 mark each for correct [H⁺] from pH (2 marks); 1 mark each for correct degree of ionisation formula and answer (2 marks). Accept ±0.5% variation.

Part (c) — Ranking (2 marks). Strongest to weakest Ka: P (HNO₃) > R (HNO₂, ~10% ionised) > S (HCOOH, ~6.3% ionised) > Q (HCN, conductivity near zero, pH barely below 7 = very high Ka). The degree of ionisation at the same concentration directly reflects Ka: higher degree = higher Ka = stronger acid. 1 mark for correct order; 1 mark for justification linking degree of ionisation to Ka.

Part (a) — Accounting for conductivity difference (3 marks). Conductivity is proportional to the total concentration of ions in solution. HCl ionises completely at all concentrations: every mol/L of HCl produces 1 mol/L H⁺ and 1 mol/L Cl⁻, giving 2 mol/L ions. As concentration doubles, conductivity doubles (linear relationship). CH₃COOH ionises only partially; at 0.10 mol/L only ≈1.3% of molecules ionise. Most of the dissolved CH₃COOH remains as intact polar molecules, which contribute negligibly to conductivity. Therefore at the same concentration, HCl produces approximately 75× more ions than CH₃COOH, explaining the enormous conductivity gap. 1 mark for linking conductivity to ion concentration; 1 mark for stating HCl 100% ionised (all dissolved molecules become ions); 1 mark for stating CH₃COOH only partially ionised (most molecules remain intact), leading to far fewer ions per litre.

Part (b) — Nearly flat CH₃COOH curve (2 marks). The nearly flat curve indicates that as concentration (c) increases, the total ion concentration increases only very slightly. This occurs because as c increases, the degree of ionisation decreases (equilibrium shifts left: more molecules are present, Le Chatelier suppresses ionisation). So while there are more acid molecules per litre, a smaller fraction ionise, and these two effects nearly cancel out. The total number of ions in solution rises much more slowly than for a strong acid at the same concentration increase. 1 mark for identifying that degree of ionisation decreases as concentration increases; 1 mark for linking this to Le Chatelier or equilibrium shift left, explaining why total ion count hardly rises.

Sample response. The statement is incorrect. Acid strength is defined by the degree of ionisation in aqueous solution, quantified by Ka, the acid dissociation constant. A strong acid (e.g. HCl) ionises completely (Ka effectively infinite); a weak acid (e.g. CH₃COOH) ionises only partially (Ka = 1.8 × 10⁻&sup5; at 25°C). pH = −log[H⁺] measures only the actual hydrogen ion concentration in that specific solution — which depends on both Ka (strength) and concentration (c). Consider a concrete comparison: 0.001 mol/L HCl gives [H⁺] = 0.001 mol/L, pH = 3.0. 10 mol/L CH₃COOH gives [H⁺] ≈ 0.0134 mol/L, pH ≈ 1.87. The concentrated weak acid has a lower pH — but at any equal concentration, HCl produces far more H⁺: at 10 mol/L, HCl gives [H⁺] = 10 mol/L (pH ≈ −1), while 10 mol/L CH₃COOH gives pH ≈ 1.87. HCl is unambiguously stronger at any given concentration. pH alone is misleading because it collapses two independent variables (Ka and c) into one number. More reliable measurements of acid strength include: (1) electrical conductivity at equal concentration (strong acid gives far more ions per litre at same c); (2) Ka value directly from experiment (ICE table + measured [H⁺] at known c). Both measurements isolate Ka from the confounding effect of concentration.

Marking notes. 1 mark — defines acid strength correctly as degree of ionisation / Ka (not “how acidic it is”). 1 mark — states that pH depends on both strength and concentration and is therefore not a reliable strength indicator alone. 1 mark — provides a quantitative example demonstrating that a concentrated weak acid can have lower pH than a dilute strong acid (values required). 1 mark — uses the same-concentration comparison to show that the strong acid always gives lower pH at equal c (values required). 1 mark — identifies conductivity at equal concentration as a more reliable method, explaining why. 1 mark — reaches a clear, explicit evaluative judgement that the statement is incorrect and states what a reliable strength measure requires.