HSC Exam Practice

Sample response. Gravimetric analysis is a quantitative analytical technique that determines the amount of an analyte by converting it to a pure, insoluble precipitate of known chemical formula, collecting and drying the precipitate, and measuring its mass. The key property required is that the precipitate must be insoluble (or very sparingly soluble) to ensure all the analyte is captured as a solid.

Marking notes. 1 mark for defining it as a mass-based technique with reference to a precipitate of known formula; 1 mark for identifying insolubility (or equivalently “quantitative precipitation”) as the required property.

Sample response. 1. Dissolve the sample so the analyte is in solution. 2. Add excess precipitating agent to form an insoluble precipitate. 3. Filter the precipitate from solution using ashless filter paper. 4. Dry (or ignite) the precipitate to remove water and convert it to a stoichiometrically defined compound. 5. Weigh the dry precipitate and use stoichiometry to calculate the analyte amount.

Marking notes. Award 1 mark for steps 1–2 in correct relative order; 1 mark for step 3 (filtration); 1 mark for steps 4–5 (dry then weigh) in correct order. Award partial marks where sequence is mostly correct.

Sample response. (a) SO₄²⁻: precipitating agent is BaCl₂(aq) (or Ba(NO₃)₂(aq)); precipitate is BaSO₄(s), barium sulfate (white). (b) Cl⁻: precipitating agent is AgNO₃(aq); precipitate is AgCl(s), silver chloride (white).

Marking notes. 1 mark per correct agent + 1 mark per correct precipitate, up to 4 marks total. Accept formula or full name for both.

Sample response. The precipitating agent is added in excess to ensure that essentially all of the analyte ion reacts and is converted to the insoluble precipitate. If the reagent is added in insufficient amount, some analyte remains dissolved (incomplete precipitation), which would reduce the collected precipitate mass and lead to an underestimate of the analyte content.

Marking notes. 1 mark for “ensure complete precipitation of the analyte” or equivalent; 1 mark for consequence of insufficient reagent: some analyte stays dissolved → underestimate.

Sample response. Incomplete precipitation occurs when not all of the analyte ion converts to the insoluble precipitate; some remains dissolved. This reduces the collected mass of precipitate below the true amount, so the calculated analyte content is an underestimate. Co-precipitation occurs when foreign ions or impurities from the solution become physically trapped within the forming precipitate. This adds extra mass to the collected solid that does not correspond to the analyte, making the measured mass too high and the calculated analyte content an overestimate.

Marking notes. 1 mark for defining incomplete precipitation correctly; 1 mark for its directional effect (mass too low → underestimate); 1 mark for defining co-precipitation correctly; 1 mark for its directional effect (mass too high → overestimate).

Sample response. A known mass of irrigation drainage water is evaporated to a dried residue and dissolved in distilled water. Excess BaCl₂(aq) is added; the sulfate ions react to form BaSO₄(s): Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The precipitate is filtered, dried and weighed. The calculation path is: n(BaSO₄) = m/M; by 1:1 stoichiometry n(SO₄²⁻) = n(BaSO₄); m(SO₄²⁻) = n×M; and % sulfate = m(SO₄²⁻) / m(sample) × 100.

Marking notes. 1 mark for describing the precipitation step with correct reagent (BaCl₂ or Ba(NO₃)₂) and product (BaSO₄); 1 mark for filter/dry/weigh steps; 1 mark for calculation path (n = m/M → stoichiometry → mass or % analyte).

Part (a) — ionic equation. Ag⁺(aq) + Cl⁻(aq) → AgCl(s). [1 mark. Deduct 1 for missing state symbols if otherwise correct; award 0 for molecular equation.]

Part (b) — calculation.
n(AgCl) = 0.1432 / 143.32 = 9.991 × 10⁻⁴ mol. [1 mark]
By 1:1 mole ratio: n(Cl⁻) = 9.991 × 10⁻⁴ mol = n(NaCl). [1 mark]
m(NaCl) = 9.991 × 10⁻⁴ × 58.44 = 0.05840 g. [1 mark]
% NaCl = (0.05840 / 2.500) × 100 = 2.34%. [1 mark]

Part (c). The cheese does not meet the FSANZ standard. The calculated NaCl content (2.34%) exceeds the 2.0% maximum. [1 mark for correct decision + 1 mark for using calculated result as justification.]

Part (a). The high result in Trial 3 is caused by incomplete drying: the precipitate was removed from the oven before it reached constant mass, so residual water was included in the balance reading. The measured precipitate mass is therefore too high, causing the calculated Ba²⁺ content to be overestimated. [2 marks: 1 for naming the error (incomplete drying); 1 for correct direction (mass too high → overestimate).]

Part (b). Valid trials: 1, 2, 4 (masses 184, 187, 188 mg).
Average mass = (184 + 187 + 188) / 3 = 186.3 mg = 0.1863 g.
n(BaSO₄) = 0.1863 / 233.39 = 7.982 × 10⁻⁴ mol.
By 1:1 ratio: n(Ba²⁺) = 7.982 × 10⁻⁴ mol.
m(Ba²⁺) = 7.982 × 10⁻⁴ × 137.33 = 0.1096 g.
% Ba²⁺ = (0.1096 / 0.400) × 100 = 27.4%.
[3 marks: 1 for correct average valid mass and identification of valid trials; 1 for n(BaSO₄) and n(Ba²⁺); 1 for m(Ba²⁺) and % with correct denominator.]

Sample Band 6 response.

Gravimetric analysis determines the mass of an analyte by converting it to a pure, insoluble precipitate of known formula, which is then collected, dried and weighed. In the industrial context of Olympic Dam mine, copper could be determined by precipitating Cu²⁺ as a copper salt (e.g. copper oxalate), igniting it to a stable oxide, and weighing the product. The calculation path — n(precipitate) = m/M; stoichiometry links to n(Cu); m(Cu) = n×M; % Cu = m(Cu)/m(sample) × 100 — provides a primary, mass-based quantitative result that does not depend on instrument calibration or spectral response.

Two significant sources of error affect reliability. First, co-precipitation: ore matrices like Olympic Dam contain multiple metal ions (Ba²⁺, Fe³⁺, Pb²⁺) that can become physically trapped in the Cu precipitate during crystal growth. This adds foreign mass, making the measured precipitate heavier than it should be, and causes the calculated Cu content to be overestimated. The analyst should wash the precipitate thoroughly with dilute acid and use a slow, controlled precipitation to minimise co-precipitation. Second, incomplete precipitation: if the precipitating agent is not in sufficient excess, some Cu²⁺ remains dissolved; the collected solid is lighter than the true precipitate mass, so Cu content is underestimated. Ensuring the reagent is added in genuine excess and confirming that no further precipitate forms on adding another drop of reagent addresses this.

Gravimetric analysis is suitable for industrial copper determination when: replicate trials agree within an acceptable tolerance; outliers are identified with a stated procedural reason and excluded; and the precipitate is dried to constant mass before weighing. Under these conditions, the technique provides a defensible, primary standard that can be used to determine ore grade, compare concentrates from different processing streams, and set commercial specifications for smelter contracts. Its limitation is speed: drying to constant mass takes hours, making it unsuitable for real-time process control, where XRF or ICP methods are faster. Nevertheless, for quality-control verification and compliance testing, gravimetric analysis remains a reliable and inexpensive reference method.

Marking criteria.

• 1 mark — Describes the principle of gravimetric analysis: analyte → insoluble precipitate of known formula → dry → weigh → stoichiometric calculation.
• 1 mark — Names and correctly describes co-precipitation: impurities trapped → mass too high → overestimate.
• 1 mark — Names and correctly describes a second error source with directional effect (incomplete precipitation → mass too low → underestimate; or incomplete drying → mass too high → overestimate; or filtration loss → mass too low → underestimate).
• 1 mark — Identifies at least one control or precaution to address a named error (e.g. excess reagent for incomplete precipitation; washing for co-precipitation; heating to constant mass for incomplete drying).
• 1 mark — States conditions under which the technique provides reliable results (consistent replicates; outliers excluded with reason; constant-mass drying).
• 1 mark — Places the technique in the industrial context: specific reference to ore grade determination, commercial specification, or comparison of processing streams at Olympic Dam or equivalent.
• 1 mark — Reaches an evaluative judgement (not just a description) of suitability: acknowledges both the strength (primary mass standard, inexpensive, no instrument calibration) and a relevant limitation (time, not suitable for real-time control), supported with reasoning.