HSC Exam Practice

Sample response. Gravimetric analysis is a quantitative analytical technique that determines the amount of a substance by measuring the mass of a product — typically a dried precipitate — formed in a chemical reaction. The precipitate must be insoluble because: (1) an insoluble solid can be physically separated from the solution by filtration with high efficiency — a soluble product would remain dissolved and could not be collected; (2) after drying, the mass of the insoluble solid reflects only the target substance, giving an accurate measurement free of solvent interference.

Marking notes. 1 mark for a correct definition of gravimetric analysis (quantitative; mass of dried precipitate used). 1 mark for explaining why insolubility is required (can be separated by filtration). 1 mark for explaining how insolubility ensures mass accuracy (dried solid mass = target only, no solvent contribution).

Sample response. (a) Filtration; key property: particle size difference — sand is an insoluble solid retained by filter paper while dissolved salt and water pass through. (b) Fractional distillation; key property: small boiling point difference (78 °C vs 97 °C, only 19 °C apart) — the fractionating column allows repeated vaporisation–condensation cycles to achieve high-purity separation. (c) Gravimetric analysis; key property: Ba²⁺ forms an insoluble precipitate (BaSO₄) with a suitable reagent; the precipitate mass is used with stoichiometry to calculate the original Ba²⁺ concentration.

Marking notes. 1 mark per correct technique (a) filtration; (b) fractional distillation [accept simple distillation if justification references the moderate boiling point difference, but fractional is the better answer for high-purity]; (c) gravimetric analysis. 1 mark per correct key property for each. Total 6 marks (1 technique + 1 property × 3).

Sample response. Excess precipitating reagent is used to ensure that all of the target ions are forced to react and precipitate from solution. The reaction is driven to completion by Le Chatelier’s principle: adding more reagent shifts the equilibrium toward further precipitation. If an insufficient amount of reagent is added, some target ions remain dissolved in the filtrate and are not collected in the precipitate. This means the measured mass of precipitate is lower than it should be, so the calculated mass/concentration of the target substance is an underestimate.

Marking notes. 1 mark for explaining that excess ensures all target ions precipitate (complete precipitation). 1 mark for explaining the consequence of insufficient reagent (some ions remain in solution / incomplete precipitation). 1 mark for stating the direction of error (result is an underestimate / mass is too low).

Sample response. Step 1: Add excess BaCl₂ solution to the 250 mL water sample; Ba²⁺ reacts with SO₄²⁻ to form white BaSO₄ precipitate [reaction: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)]. Step 2: Filter the precipitate through ashless filter paper; this separates the insoluble BaSO₄ from the solution and uses ashless paper to avoid adding extra mass when the paper is later ashed. Step 3: Wash the precipitate on the filter paper with distilled water; this removes soluble impurity ions (e.g. Ba²⁺, Cl⁻) that could add extra mass. Step 4: Dry the precipitate to constant mass in an oven (or desiccator); weigh on an analytical balance; use stoichiometry (n = mass ÷ M(BaSO₄); n(SO₄²⁻) = n(BaSO₄) from 1:1 ratio; m(SO₄²⁻) = n × M(SO₄²⁻)) to calculate the mass of sulfate in the original sample.

Marking notes. 1 mark per correctly described key step: (1) add excess BaCl₂, precipitation occurs; (2) filter with ashless paper; (3) wash with distilled water; (4) dry to constant mass, weigh, apply stoichiometry. Award 4 marks for all four steps correctly described in the right order with a justification for at least two steps.

Sample response. Both techniques use differences in boiling point to separate liquids, but they are suited to different situations. Simple distillation is used when the boiling point difference between the two components is large (typically >25 °C) or when separating a volatile liquid from a non-volatile dissolved solid; for example, distilling tap water away from dissolved minerals to produce distilled water. Fractional distillation is used when the boiling points are close together (typically <25 °C) and both components are volatile; it uses a fractionating column to provide repeated vaporisation–condensation cycles that gradually enrich the vapour in the more volatile component; for example, separating crude oil fractions at an Australian petroleum refinery (e.g. Geelong refinery), or separating ethanol (78 °C) from propanol (97 °C) to obtain a pure product.

Marking notes. 1 mark for correct definition of simple distillation (large BP difference or non-volatile solute) with a valid example. 1 mark for correct definition of fractional distillation (close BPs; fractionating column; repeated cycles) with a valid example. 1 mark for clearly distinguishing when each is chosen (the “when” criterion). 1 mark for two valid named examples (one for each technique).

Sample response. The precipitate must be dried to constant mass rather than for a set time because different precipitates absorb different amounts of water and dry at different rates; a fixed drying time may be insufficient for some precipitates, leaving residual moisture. “Constant mass” means the mass no longer decreases with further drying — i.e. all water and volatile impurities have been removed. A student confirms constant mass by: (1) drying the precipitate in an oven, (2) cooling it in a desiccator (to prevent reabsorption of atmospheric moisture), (3) weighing it, (4) drying for a further period and reweighing. If two consecutive masses agree to the precision of the balance, constant mass has been reached. Any residual moisture would add falsely to the measured precipitate mass, causing an overestimate of the target substance concentration.

Marking notes. 1 mark for explaining why a fixed time is unreliable and defining constant mass (mass no longer changes). 1 mark for describing the procedure to confirm constant mass (dry → cool in desiccator → weigh → repeat until consecutive masses agree). 1 mark for stating the effect of residual moisture (overestimate of the target substance).

Sample response (a). Net ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). [1:1 molar ratio]

Sample response (b). n(BaSO₄) = mass ÷ M = 0.932 ÷ 233.4 = 3.993 × 10⁻³ mol ≈ 3.99 × 10⁻³ mol.

Sample response (c). From the balanced equation, n(SO₄²⁻) = n(BaSO₄) = 3.99 × 10⁻³ mol (1:1 molar ratio).

Sample response (d). m(SO₄²⁻) = n × M = 3.99 × 10⁻³ × 96.1 = 0.384 g (to 3 significant figures).

Sample response (e). c(SO₄²⁻) = 0.384 g ÷ 0.500 L = 0.768 g/L = 768 mg/L.

Sample response (f). Assumption: all of the precipitate mass comes from BaSO₄ formed from SO₄²⁻ ions only — i.e. no other ions co-precipitate with Ba²⁺. If another ion (e.g. carbonate, CO₃²⁻) were present and precipitated as BaCO₃ (also insoluble), the measured precipitate mass would be higher than the mass of BaSO₄ alone. Since the calculation treats all of the precipitate as BaSO₄, this would lead to an overestimate of the SO₄²⁻ concentration.

Marking notes. (a) 1 mark for correct ionic equation with state symbols. (b) 1 mark for correct moles of BaSO₄. (c) 1 mark for applying 1:1 ratio correctly. (d) 1 mark for correct mass of SO₄²⁻ to correct sig figs. (e) 1 mark for correct concentration in mg/L. (f) 1 mark for a clearly stated assumption (precipitate is pure BaSO₄ from SO₄²⁻ only); 1 mark for explaining how co-precipitation causes an overestimate.

Sample response. Gravimetric analysis is a highly effective quantitative technique for determining specific ion concentrations in water, widely used in Australian water quality monitoring (e.g. Sydney Water, the NSW EPA, and mine-site environmental monitoring near Newcastle or Broken Hill). Its core strength is accuracy: by using a direct mass measurement of a dried, pure precipitate on a calibrated analytical balance, the technique avoids the need for colour charts, light absorbance readings, or calibration curves that introduce additional uncertainty. It is highly reproducible and can detect very low concentrations of ions (milligrams per litre), provided the precipitate is sufficiently insoluble and forms quantitatively. Each procedural step directly ensures accuracy: adding excess reagent drives precipitation to completion; ashless filter paper avoids adding extra mass; washing removes co-precipitated impurities; drying to constant mass removes all moisture so the final mass reflects only the target compound; and stoichiometry bridges the measurable quantity (precipitate mass) to the desired quantity (original ion mass).

However, the technique has significant limitations. It is slow (drying to constant mass can take hours) and labour-intensive, making it impractical for high-throughput routine testing. Sea water and industrial waste water often contain interfering ions (e.g. Br⁻, CO₃²⁻, PO₄³⁻) that co-precipitate with the target, inflating the measured mass and causing systematic overestimation — this is the most serious limitation in complex matrices. The technique is also specific to ions that form suitable insoluble precipitates; it cannot directly measure, for example, dissolved organic pollutants or pH. In comparison, ion chromatography (used at the ANSTO Lucas Heights laboratory and major water utilities) can simultaneously measure multiple anions (Cl⁻, SO₄²⁻, NO₃⁻) in a single run in under 30 minutes with similar detection limits, at a lower labour cost per sample but a higher instrument capital cost. For a one-off highly accurate measurement of a single ion in a clean matrix (e.g. chloride in rain water), gravimetric analysis remains the most accurate and accessible method. For routine multi-ion monitoring of complex water sources, ion chromatography or atomic absorption spectroscopy is more practical.

Overall, gravimetric analysis is most effective when: accuracy is the highest priority; the matrix is relatively simple (few interfering ions); and speed is not a constraint. Its step-by-step precision makes it a benchmark method, but its limitations in complex matrices and slow throughput mean it is increasingly supplemented by instrumental techniques in modern Australian water testing laboratories.

Marking criteria (7 marks). 1 = identifies and explains at least two genuine strengths of gravimetric analysis (e.g. accuracy, direct mass measurement, no calibration curve needed, sensitivity). 1 = identifies and explains at least two genuine limitations (e.g. slow, co-precipitation, labour-intensive, only works for ions forming insoluble precipitates). 1 = explains the purpose of at least two specific procedural steps and links each to accuracy (e.g. excess reagent → complete precipitation; drying to constant mass → removes moisture overestimate). 1 = names and correctly describes at least one alternative technique (e.g. ion chromatography, AAS, potentiometric titration) and explains how it compares. 1 = named Australian context used correctly (e.g. NSW EPA, Sydney Water, mine-site monitoring, ANSTO). 1 = the response contrasts a situation where gravimetric analysis is most appropriate with one where an alternative is better (environment-dependent judgement). 1 = reaches an explicit evaluative conclusion integrating both strengths and limitations, not a one-sided answer.