Chemistry • Year 12 • Module 8 • Lesson 2

Gravimetric Analysis

Develop HSC Band 5–6 extended-response technique: synthesise data with method critique and evaluate a flawed claim about gravimetric analysis.

Master · Extended Response (Band 5–6)

1. Data + scenario: BHP Olympic Dam copper determination (Band 5–6)

8 marks

Context. BHP’s Olympic Dam mine in South Australia produces copper concentrate from a copper sulfide ore body. In the mine’s analytical laboratory, a quality-control chemist determines the copper content of a concentrate sample using gravimetric analysis. The copper (Cu²⁺) is first oxidised and then precipitated as copper(II) oxalate, CuC₂O₄(s), before ignition converts it to CuO(s) for final weighing. Three replicate 0.650 g samples are analysed. Results:

Replicate	m(sample) (g)	m(CuO precipitate) (g)	Note
1	0.650	0.221	Procedure followed correctly
2	0.650	0.219	Procedure followed correctly
3	0.650	0.248	Co-precipitation noted: barium sulfate impurity present in ore matrix trapped with precipitate

M(CuO) = 79.55 g mol⁻¹; M(Cu) = 63.55 g mol⁻¹. Equation: Cu²⁺(aq) + C₂O₄²⁻(aq) → CuC₂O₄(s) → (on ignition) CuO(s). Mole ratio Cu : CuO = 1:1.

Q1. Using the data above, evaluate the reliability of these gravimetric results and determine the percentage of copper in the concentrate. In your response you must:

Identify which replicate(s) should be excluded and justify this with reference to a named source of error and its directional effect on the measured mass.
Use the valid replicate(s) to calculate the percentage by mass of copper (Cu) in the original 0.650 g sample. Show all steps.
Explain how the mine would use this result in an industrial decision-making context.
Describe one other precaution the laboratory should take during filtration to improve accuracy, and link it to a specific error source.

Stuck? Plan: (1) identify outlier & error direction → (2) average valid replicates → (3) n(CuO) = m/M → n(Cu) = same (1:1) → m(Cu) = n×M(Cu) → % → (4) industrial context → (5) filtration precaution.

2. Source critique — evaluate a claim about gravimetric analysis (Band 5–6)

7 marks

“Gravimetric analysis is an outdated and unreliable technique for determining analyte content. Because it relies on physical mass measurements, random errors in the balance are the only significant source of inaccuracy. A careful analyst can eliminate all errors by simply weighing the precipitate more precisely. Furthermore, because the technique involves precipitation, it will overestimate the analyte content every time — so all gravimetric results must be corrected downward before reporting.”

Attributed to: Year 12 student summary, practice notes.

Q2. Evaluate this claim. Identify what is scientifically correct, what is incorrect or oversimplified, and rewrite the claim as a biologically and chemically defensible statement using precise lesson terminology.

Stuck? Identify each sentence in the claim and check it against Cards 4 and 5. Does precipitation always overestimate? What are the real sources of error? Can they all be eliminated by weighing precision?

Answers — Do not peek before attempting

Q1 — Sample Band 6 response (8 marks), annotated

Step 1 — Identify and justify exclusion. Replicate 3 should be excluded because of co-precipitation: the note states that barium sulfate impurity from the ore matrix became trapped within the CuO precipitate. Co-precipitation adds mass from a foreign compound, making the measured precipitate mass too high. This leads to an overestimate of copper content. Replicates 1 and 2 follow the correct procedure and are consistent (0.221 g and 0.219 g), so only they are averaged. [2 marks: 1 for identifying co-precipitation by name and 1 for correct direction (mass too high → overestimate)]

Step 2 — Calculation.

Average valid CuO mass = (0.221 + 0.219) / 2 = 0.2200 g.

n(CuO) = m / M = 0.2200 / 79.55 = 2.766 × 10⁻³ mol.

By 1:1 mole ratio: n(Cu) = 2.766 × 10⁻³ mol.

m(Cu) = n × M = 2.766 × 10⁻³ × 63.55 = 0.1757 g (accept 0.176 g).

% Cu = (0.176 / 0.650) × 100 = 27.1% (accept 27.0–27.2%). [3 marks: 1 for correct n(CuO), 1 for correct n(Cu) and m(Cu), 1 for % with correct sample mass]

Step 3 — Industrial context. The mine uses the copper percentage to determine the grade of concentrate: a result near 27% Cu would be compared against target specification (Olympic Dam typically aims for >25% Cu in concentrate). If grade is below specification, the ore blending or flotation circuit may need adjustment. Gravimetric data provides a defensible, primary mass-based standard that can be cited in contracts with smelters. [1 mark for a specific industrial decision linked to the calculated percentage]

Step 4 — Filtration precaution. The analyst should use ashless filter paper (quantitative filtration) and ensure the precipitate is washed with dilute acid to remove adsorbed ions. This specifically addresses co-precipitation: washing dislodges surface-adsorbed impurities, reducing the foreign-ion contribution to the final mass. [1 mark for a named precaution linked to a named error source with directional reasoning]

Marking criteria.

1 mark — Correctly identifies Replicate 3 as the outlier by name (co-precipitation noted in the table).
1 mark — Correctly explains the directional effect of co-precipitation (mass too high → analyte overestimated).
1 mark — Correctly calculates n(CuO) from the average valid mass using n = m/M (award even if arithmetic is slightly off but method is correct).
1 mark — Correctly converts to m(Cu) using the 1:1 mole ratio and M(Cu) = 63.55 g mol⁻¹.
1 mark — Correctly states % Cu with working, using the 0.650 g sample mass as the denominator.
1 mark — Makes a specific, contextualised industrial decision using the copper percentage (e.g. grade comparison, circuit adjustment, commercial specification).
1 mark — Names a valid filtration precaution (e.g. ashless filter paper, washing precipitate with dilute acid) and links it explicitly to a named error source.
1 mark — Overall response uses precise lesson terminology throughout (analyte, co-precipitation, stoichiometry, percentage composition or % by mass).

Q2 — Sample Band 6 source critique (7 marks)

The claim contains one defensible element, multiple significant errors, and a wholly incorrect generalisation. [1 — overall evaluative judgement]

What is defensible: It is true that gravimetric analysis involves a mass measurement and that balance precision is one contributor to the final result. More precise weighing does reduce random error from the instrument. However, this is a minor concession. [1 — correctly concedes a valid element]

What is incorrect (1): “random balance errors are the only significant source of inaccuracy.” This is wrong. Gravimetric analysis has at least four significant chemical and procedural error sources that are completely independent of the balance: incomplete precipitation (analyte remains dissolved), co-precipitation (impurities trapped in precipitate), incomplete drying (water inflates mass), and filtration losses (precipitate physically lost). None of these are addressed by weighing more precisely. [1 — correctly refutes “only balance errors matter” with named alternatives]

What is incorrect (2): “a careful analyst can eliminate all errors.” Careful technique minimises but cannot eliminate all chemical sources of error. Co-precipitation and slight solubility of the precipitate are thermodynamic phenomena that persist regardless of how carefully the analyst works; they can only be reduced, not removed. [1 — correctly distinguishes “reduce” from “eliminate” with mechanism]

What is incorrect (3): “precipitation will overestimate the analyte every time.” This is wrong. Several errors cause underestimation: incomplete precipitation (some analyte stays dissolved) and filtration losses (some precipitate physically lost) both reduce the measured mass below the true value. Only co-precipitation and incomplete drying cause overestimation. There is no directional guarantee. [1 — correctly refutes the “always overestimates” claim with named counterexamples]

Defensible reformulation: “Gravimetric analysis is a well-established quantitative technique that determines analyte content from the mass of a pure, dry precipitate of known formula. Its accuracy depends on careful technique at each step, not only on weighing precision. Errors can arise from incomplete precipitation or filtration losses (causing underestimation) or from co-precipitation and incomplete drying (causing overestimation). A reliable result requires replicate trials, exclusion of outliers with a stated reason, and procedural controls at each step of the dissolve–precipitate–filter–dry–weigh sequence.” [1 — defensible reformulation using precise lesson terminology and correct directional language]

Marking criteria.

1 mark — States an overall evaluative judgement (e.g. “partially correct but largely flawed” or equivalent).
1 mark — Correctly acknowledges the one defensible element (balance precision does reduce random error / mass measurement is central).
1 mark — Correctly refutes “only balance errors matter” by naming at least two chemical/procedural error sources (from: incomplete precipitation, co-precipitation, incomplete drying, filtration losses).
1 mark — Correctly argues that errors cannot all be “eliminated” — some are thermodynamically inherent (e.g. slight solubility, co-precipitation); careful technique only minimises them.
1 mark — Correctly refutes “always overestimates” by identifying at least one error source that causes underestimation (incomplete precipitation or filtration loss) with directional reasoning.
1 mark — Reformulates the claim into a biologically defensible statement that retains the correct element and corrects the flawed ones.
1 mark — Throughout the response, uses precise lesson terminology (analyte, co-precipitation, incomplete precipitation, stoichiometry, percentage composition, direction of error).