HSC Exam Practice

Sample response. An indicator is a weak acid (HIn) whose acid form (HIn) and conjugate base form (In–) have different colours. The equilibrium HIn ⇌ H¹ + In– shifts depending on the [H¹] of the solution: in acidic solution the equilibrium lies left (HIn dominates, one colour); in basic solution it shifts right (In– dominates, different colour). This colour change signals the endpoint.

Marking notes. 1 mark: indicator is a weak acid (HIn ⇌ H¹ + In–). 1 mark: acid and conjugate base forms have different colours. 1 mark: colour change occurs when the equilibrium shifts in response to pH change (or explicit mention of H¹ concentration shifting the equilibrium).

Sample response. The equivalence point is where stoichiometrically equivalent amounts of acid and base have reacted; its pH depends on the salt formed. The endpoint is the observed colour change of the indicator. If the indicator's transition range does not include the equivalence point pH, the colour change occurs at a different pH (e.g. in the buffer region or after equivalence), so the student stops the titration at a volume far from the true equivalence volume, causing a systematic error in the calculated concentration.

Marking notes. 1 mark: correct definition of equivalence point. 1 mark: correct definition of endpoint. 1 mark: explaining how an incorrect indicator makes the endpoint occur at a pH different from the equivalence point pH, causing error in the titre.

Sample response. (a) Strong acid / strong base: EP pH = 7; any of the three indicators is valid (large pH jump ~4–10 encompasses all); bromothymol blue or phenolphthalein are common choices. (b) Weak acid / strong base: EP pH > 7 (~8.7 for CH₃COOH/NaOH); phenolphthalein (8.3–10.0) — its range encompasses the EP pH. (c) Strong acid / weak base: EP pH < 7 (~5.3 for HCl/NH₃); methyl orange (3.1–4.4) — the sharp pH jump in the acidic region is within MO's range.

Marking notes. 1 mark per correctly named indicator with EP pH direction stated or justified (max 3). No mark for indicator only without justification.

Sample response. At the equivalence point, the salt of a weak acid and strong base (e.g. CH₃COONa) is present. The conjugate base ion (CH₃COO–) partially hydrolyses: CH₃COO– + H₂O ⇌ CH₃COOH + OH–. The production of OH– makes the solution basic, so pH > 7.

Marking notes. 1 mark: identifies the conjugate base ion as the species present at EP. 1 mark: correct hydrolysis equation with OH– product. 1 mark: concludes that OH– production gives pH > 7.

Sample response. For a weak acid / weak base titration, there is no sharp pH jump at the equivalence point — the pH changes gradually over many mL of added base. Any indicator would change colour gradually over a wide volume range, making it impossible to identify a precise endpoint. The error in the titre would be too large to be useful.

Marking notes. 1 mark: no sharp pH jump at equivalence. 1 mark: colour change is gradual / no precise endpoint can be located.

Sample response. Using the Henderson–Hasselbalch equation for the indicator: pH = pK_a(In) + log([In–]/[HIn]). The eye perceives predominantly the HIn colour when [HIn]/[In–] ≥ 10 (i.e. pH ≤ pK_a – 1) and predominantly the In– colour when [In–]/[HIn] ≥ 10 (i.e. pH ≥ pK_a + 1). The transition range — where both species are present in comparable amounts and a mixed colour is visible — therefore spans approximately pK_a(In) – 1 to pK_a(In) + 1, a range of about 2 pH units.

Marking notes. 1 mark: states that the transition occurs where both HIn and In– are present in significant concentrations. 1 mark: explains that the eye favours one colour when the ratio is ≥ 10:1 (or equivalent). 1 mark: concludes that the range spans approximately ± 1 pH unit around pK_a(In).

(a) Indicator choice — 2 marks. Phenolphthalein (8.3–10.0). From the graph, the EP pH ≈ 8.9 and the sharp pH jump runs approximately pH 7.5–12. Phenolphthalein's band (purple/violet shading) overlaps this jump. BTB (6.0–7.6) falls below the jump; MO (3.1–4.4) falls in the buffer region. (1 mark indicator; 1 mark correct graph-based justification.)

(b) Concentration — 3 marks. n(NaOH) at EP = 0.100 × 0.02000 = 2.00 × 10–³ mol = n(acid). c(acid) = 2.00 × 10–³ / 0.02500 = 0.0800 mol/L. (1 mark: n(NaOH); 1 mark: equimolar ratio; 1 mark: correct concentration.)

(c) pKa — 2 marks. Half-equivalence volume = 20.00 / 2 = 10.00 mL. At this volume, [acid] = [conjugate base], so log([A–]/[HA]) = 0, therefore pH = pK_a. From graph at 10.00 mL: pH ≈ 4.8. K_a = 10–⁴‧⁸ ≈ 1.6 × 10–⁵. (1 mark reasoning; 1 mark K_a value or correct pK_a from graph.)

(d) BTB outcome — 3 marks. From the graph, BTB's range (6.0–7.6) intersects the gradually rising section of the curve before the sharp jump, at approximately 17–18 mL NaOH. Using BTB, the student would observe a colour change at ≈17–18 mL, recording an endpoint 2–3 mL before the true equivalence point (20.00 mL). The calculated concentration of the weak acid would be underestimated. (1 mark: BTB falls below EP; 1 mark: endpoint recorded early from graph; 1 mark: consequence — titre too small / concentration underestimated.)

(a) Hydrolysis equation — 2 marks. At equivalence, HCOONa is present. Hydrolysis: HCOO– + H₂O ⇌ HCOOH + OH–. OH– is produced → basic solution → EP pH > 7. (1 mark equation; 1 mark reasoning.)

(b) EP pH calculation — 4 marks. At EP: equal volumes mixed → [HCOO–] = 0.0500 mol/L.
K_b(HCOO–) = K_w/K_a = 1.0 × 10–¹´ / 1.77 × 10–´ = 5.65 × 10–¹¹.
[OH–] = √(5.65 × 10–¹¹ × 0.0500) = √(2.82 × 10–¹²) = 1.68 × 10–⁶ mol/L.
pOH = –log(1.68 × 10–⁶) = 5.77. EP pH = 14.00 – 5.77 = 8.23.
(1 mark: [HCOO–] after dilution; 1 mark: K_b; 1 mark: [OH–]; 1 mark: pH = 8.23.)

(c) Indicator — 1 mark. Phenolphthalein (8.3–10.0) — EP pH 8.23 is at the lower edge of phenolphthalein's range; the sharp pH jump encompasses this range and the student will observe a clear colour change close to equivalence. (Accept phenolphthalein with correct reasoning. Note: BTB would also give a colour change close to EP in this case but is below the EP pH; phenolphthalein is the more reliable choice.)

Sample response. The claim is incorrect: indicator selection critically determines the accuracy of an acid-base titration result. Indicators are weak acids (HIn) in equilibrium: HIn ⇌ H¹ + In–. The acid form carries one colour; the conjugate base a different colour. The transition pH range is approximately pK_a(In) ± 1. For a result to be accurate, the indicator's range must overlap the sharp pH jump that occurs at the equivalence point — not simply any region of the curve. The equivalence point pH is not universally pH 7: it is determined by the salt formed. For a weak acid / strong base titration (e.g. ethanoic acid + NaOH), the conjugate base CH₃COO– hydrolyses to produce OH–, giving an EP pH of approximately 8.7. Phenolphthalein (range 8.3–10.0) is valid here; methyl orange (range 3.1–4.4) is not. If methyl orange is used for this titration, the colour change occurs in the buffer region — at approximately pH 3.7–4.4 — when only ~10–20% of the acid has been neutralised. The recorded titre would be far smaller than the true equivalence volume, causing a severe underestimate of the calculated acid concentration. In an analytical chemistry laboratory context (e.g. NATA-accredited testing for acidity of wine as per AWRI protocols), such an error would render the result worthless and potentially dangerous. Therefore, indicator choice is not a minor detail — it is the variable that determines whether the endpoint coincides with the equivalence point, and an incorrect choice introduces a large, systematic, non-random error that cannot be detected by replication alone.

Marking notes. 1 mark: correct mechanism (HIn ⇌ H¹ + In–; different colours; equilibrium shifts with pH). 1 mark: defines equivalence point with correct EP pH for at least one specific titration type. 1 mark: explains that the indicator range must overlap the sharp pH jump at equivalence (not just any region). 1 mark: names at least two indicators with correct transition ranges. 1 mark: specific quantified example of error from wrong indicator (direction and mechanism of error). 1 mark: applies real-world context (analytical lab, AWRI, QC, NATA, or equivalent). 1 mark: clear evaluative conclusion explicitly rejecting the claim with reasoned justification.