HSC Exam Practice

Sample response. The equivalence point is the point in a titration at which stoichiometrically equivalent amounts of acid and base have reacted, as defined by the mole ratio in the balanced equation.

Marking notes. 1 mark for “stoichiometrically equivalent amounts of acid and base have reacted”; 1 mark for link to the balanced equation / mole ratio (not just “equal volumes” or “pH 7”).

Sample response. The equivalence point is a chemical concept: the point at which stoichiometric neutralisation is complete. The endpoint is an experimental observation: the point at which the indicator changes colour. A well-chosen indicator makes these two points coincide closely, but they are not identical.

Marking notes. 1 mark for equivalence = stoichiometric completion (a chemical quantity); 1 mark for endpoint = indicator colour change (an experimental signal). Penalise answers that say they are “always the same”.

Sample response. Methyl orange (pH 3.1–4.4) is the most suitable indicator. HCl is a strong acid and NH₃ is a weak base, so the equivalence point is below pH 7 (the conjugate acid NH₄⁺ makes the solution acidic at equivalence, typically around pH 4–5). Methyl orange transitions in the acidic range and its transition region overlaps the steep pH drop near the equivalence point, giving an endpoint that is close to the true equivalence point.

Marking notes. 1 mark for correctly identifying methyl orange; 1 mark for stating the equivalence point is below pH 7 for strong acid–weak base; 1 mark for explaining that the indicator’s transition range overlaps the steep region near the equivalence point.

Sample response. Perform a rough titration first to locate the approximate endpoint region. Then perform replicate titrations; collect titres that agree within 0.10 mL of each other — these are the concordant titres. Calculate the mean of the concordant titres (excluding the rough and any outliers) and use this value in the n = cV calculation to determine the unknown concentration.

Marking notes. 1 mark for performing and excluding the rough trial; 1 mark for defining concordant as within 0.10 mL and averaging those values; 1 mark for using the average titre in n = cV to find the unknown concentration.

Sample response. Antacid tablets are solids that may dissolve slowly and incompletely, making it difficult to deliver a precise aliquot for direct titration. Additionally, the base in a tablet may react sluggishly, making the endpoint unclear. In back titration, a known excess of HCl is added to ensure the reaction goes to completion; the excess HCl is then measured by a second titration with NaOH. This allows the amount of base that reacted to be calculated accurately by subtraction.

Marking notes. 1 mark for “solid sample cannot be easily aliquoted / may dissolve slowly”; 1 mark for “known excess of HCl ensures complete reaction”; 1 mark for “excess is measured in a second titration and subtracted from initial moles to find moles reacted with the tablet”.

Sample response. (a) Strong acid + weak base: the equivalence point is below pH 7 because the weak-base conjugate acid makes the solution acidic at equivalence. Methyl orange (pH 3.1–4.4) is the correct choice because its transition range overlaps the steep pH drop in the acidic region near the equivalence point. (b) Weak acid + strong base: the equivalence point is above pH 7 because the salt produced is basic. Phenolphthalein (pH 8.2–10.0) is the correct choice because it transitions in the basic region where the steep pH jump occurs. (c) Strong acid + strong base: the equivalence point is at pH 7 and the steep jump is large, spanning roughly pH 4–10. Any of the three indicators listed — methyl orange, bromothymol blue, or phenolphthalein — can work acceptably, but bromothymol blue (pH 6.0–7.6) gives the endpoint closest to pH 7.

Marking notes. 1 mark for correct indicator for strong acid + weak base with a reason; 1 mark for correct indicator for weak acid + strong base with a reason; 1 mark for correct indicator(s) for strong acid + strong base with a reason; 1 mark for linking the choice in each case to the pH at the equivalence point and the indicator transition range.

Sample response (a). Trials 1 and 2 are concordant (both 24.95 mL; spread = 0.00 mL, within 0.10 mL). Trial 3 (25.05 mL) is excluded because it differs from Trials 1 and 2 by 0.10 mL, placing it just outside the concordant group. The rough trial is excluded as it was used only to locate the endpoint. Average concordant titre = (24.95 + 24.95) / 2 = 24.95 mL.

Sample response (b). n(KHP) = cV = 0.0500 × 0.02500 = 1.250 × 10⁻³ mol. The mole ratio KHP : NaOH = 1:1, so n(NaOH) = 1.250 × 10⁻³ mol. V(NaOH) = 24.95 mL = 0.02495 L. c(NaOH) = n/V = 1.250 × 10⁻³ / 0.02495 = 0.05010 mol L⁻¹.

Marking notes (a). 1 mark for identifying Trials 1 and 2 as concordant with correct average; 1 mark for justified exclusion of Trial 3 (0.10 mL outside concordant group) and rough. (b) 1 mark for n(KHP) correct; 1 mark for applying 1:1 ratio; 1 mark for correct c(NaOH).

Sample response (a). n(NaOH) = 0.1000 × 0.02250 = 0.002250 mol. NaOH and HCl react 1:1, so n(excess HCl) = 0.002250 mol. n(initial HCl) = 0.1500 × 0.04000 = 0.006000 mol. n(HCl reacted with Mg(OH)₂) = 0.006000 − 0.002250 = 0.003750 mol. Mg(OH)₂ : HCl = 1:2, so n(Mg(OH)₂) = 0.003750 / 2 = 0.001875 mol. m = nM = 0.001875 × 58.32 = 0.1094 g (= 0.109 g to 3 s.f.).

Sample response (b). The rough titre is generally larger than the concordant titres (the student overshoots on the first run). If a larger titre were used, n(excess HCl) would be larger, so the calculated n(HCl that reacted with the tablet) would be smaller, and the calculated mass of Mg(OH)₂ would be lower than the true value.

Marking notes (a). 1 mark for n(NaOH) correct; 1 mark for subtracting excess from initial HCl; 1 mark for applying 1:2 ratio; 1 mark for correct mass 0.109 g. (b) 1 mark for stating “lower”; 1 mark for correct reasoning (rough titre larger → larger excess HCl calculated → less HCl appears to have reacted → smaller calculated Mg(OH)₂).

Sample response. Titration is a quantitative analytical technique used to determine the unknown concentration of an acid or base by reacting it with a standard solution of known concentration. Its accuracy depends on three key components: the quality of the standard solution, the handling of titre data, and the choice of indicator.

The standard solution must itself be verified against a primary standard — a highly pure, stable solid (such as Na₂CO₃ or KHP) that can be accurately weighed to give a solution of precisely known concentration. Relying on the nominal label concentration of a reagent bottle is insufficient in industrial analysis because NaOH absorbs CO₂ and moisture, reducing its effective concentration over time. Using a primary standard ensures the result is traceable to a known reference, satisfying the requirements of NATA accreditation in Australian industrial laboratories such as those at Orica or the TGA.

Titre reliability requires performing multiple trials and using only concordant titres (agreeing within ±0.10 mL) in the final average. The rough trial is excluded because it is used only to locate the endpoint region. Averaging concordant results reduces the impact of random errors in individual readings, improving precision. The precision of titration is ultimately limited by the 0.05 mL graduation of the burette and the human judgement in reading the meniscus.

Indicator selection is the chemical link between the experimental endpoint and the true equivalence point. The indicator must have its pH transition range overlapping the steep portion of the titration curve near the equivalence point. For a strong acid–strong base titration the steep jump spans roughly pH 4–10, so a range of indicators is acceptable. For a weak acid–strong base titration the equivalence point is above pH 7, so phenolphthalein (pH 8.2–10.0) is required; methyl orange would change colour too early and give a titre that is too small, underestimating the acid concentration. Conversely, for a strong acid–weak base titration methyl orange is correct and phenolphthalein would cause an error by changing colour after the equivalence point is passed.

Overall, titration is a highly reliable and accurate analytical technique when all three elements are correctly implemented. Its main limitations are the visual judgement of the endpoint (which introduces a small but irreducible indicator error) and the need for a clean, compatible primary standard for the specific analyte being studied.

Marking notes. 1 mark — defines titration as determining unknown concentration via standard solution + stoichiometry. 1 mark — explains role of primary standard (pure, stable, accurately weighed; contrasts with nominal bottle label). 1 mark — explains concordant titres (within 0.10 mL; rough excluded; reduces random error). 1 mark — states the general indicator selection rule (transition range overlaps steep jump near equivalence). 1 mark — applies indicator rule to at least one specific titration type with correct indicator named. 1 mark — explains what happens when an incorrect indicator is used (early or late endpoint; effect on titre and calculated concentration). 1 mark — reaches an overall evaluative judgement about the reliability and limitations of titration as an analytical technique.

Sample response. The statement contains a significant scientific flaw: it conflates the endpoint with the equivalence point, implying that the indicator colour change identifies the exact point of complete neutralisation. This is incorrect. The indicator changes colour at the endpoint, which is an experimental signal, not the same as the equivalence point, which is the chemical reality of stoichiometric completion. A well-chosen indicator makes the endpoint occur close to the equivalence point, but they are not identical. An indicator whose transition range does not overlap the steep pH jump near the equivalence point will change colour either before or after neutralisation is complete, introducing a systematic titration error. The accuracy of a titration depends not on the indicator alone but on the combination of: a verified standard solution, concordant titres, an indicator whose transition range is within the steep pH jump, and correct technique. The report overstates the precision of visual indicator detection.

Marking notes. 1 mark for identifying the flaw: the statement equates endpoint with equivalence point. 1 mark for explaining that endpoint = indicator colour change (experimental) while equivalence point = stoichiometric completion (chemical); they should coincide but are not the same. 1 mark for explaining that a poorly chosen indicator causes the endpoint to occur before or after the equivalence point, introducing error. 1 mark for an overall evaluative statement that identifies what titration accuracy actually depends on (standard solution quality, concordant titres, correct indicator selection, technique) and explains why the statement is an oversimplification.