HSC Exam Practice

Sample response. Gravimetric analysis is a quantitative technique that determines the amount of an analyte by converting it into a pure, insoluble solid of known chemical formula and measuring the mass of that solid [1]. The precipitate plays a central role: it is the solid product formed when the precipitating reagent reacts with the analyte in solution [1]. Because the precipitate has a known formula, the mole ratio from the balanced equation allows the moles (and therefore concentration) of the original analyte to be calculated precisely from the measured precipitate mass [1].

Marking notes. 1 mark for defining gravimetric analysis (quantitative, mass-based, known formula); 1 mark for describing the precipitate as the insoluble product formed in solution; 1 mark for explaining that the known formula enables calculation via mole ratio.

Sample response. A slight excess of precipitating reagent is added to drive the precipitation reaction to completion, ensuring that essentially all the analyte ions are converted to precipitate. Without sufficient excess, some analyte would remain in solution and not be collected, giving a result that is too low [1]. However, an extremely large excess (e.g. 10× stoichiometric) should be avoided because at very high concentrations of the precipitating ion, the precipitate can form soluble complex ions, increasing its apparent solubility — this is the complex ion effect [1]. Therefore, a moderate excess (approximately 10–20% more than stoichiometric) represents the optimum: complete reaction without the risk of re-dissolving the precipitate [1].

Marking notes. 1 mark for explaining why excess is needed (drives reaction to completion); 1 mark for identifying the risk of very large excess (complex ion formation / solubility increase); 1 mark for concluding that a moderate excess is optimal.

Sample response. (1) Dissolve the water sample to bring Cl⁻ ions into aqueous solution. [1] (2) Add a slight excess of silver nitrate (AgNO₃) solution as the precipitating reagent; Ag⁺(aq) + Cl⁻(aq) → AgCl(s) forms a white precipitate. [1] (3) Filter the AgCl precipitate through pre-weighed filter paper; wash with distilled water to remove soluble impurities. [1] (4) Dry the precipitate and filter paper in an oven (~120 °C) to constant mass; cool in a desiccator before weighing. [1] (5) Weigh the cooled precipitate accurately; subtract the filter paper mass to obtain m(AgCl); use n = m ÷ MM and the 1 : 1 mole ratio to calculate n(Cl⁻) and then c(Cl⁻) = n ÷ V. [1]

Marking notes. 1 mark per clearly described step. Must name AgNO₃ as the precipitating reagent and AgCl as the precipitate for full credit on step 2. Accept equivalent wording for each step.

Sample response. Gravimetric analysis measures the mass of a precipitate using an analytical balance; volumetric analysis measures the volume of a standard solution at the equivalence point using a burette [1]. The key equipment for gravimetric analysis includes an analytical balance, oven, desiccator, and filter paper; volumetric analysis uses a burette, pipette, conical flask, and indicator [1]. Gravimetric analysis is best suited to analytes that form very insoluble precipitates (e.g. SO₄²⁻, Cl⁻), while volumetric analysis is best suited to acid–base or redox analytes that participate in reactions with a sharp endpoint [1]. Gravimetric analysis is slower (hours due to drying) but achieves very high mass precision; volumetric analysis is faster (minutes) but relies on accurate volume measurement [1].

Marking notes. 1 mark for contrasting what is measured; 1 mark for contrasting equipment; 1 mark for contrasting best-suited analytes; 1 mark for an additional contrasting feature (speed, precision, or error source).

Sample response. Error 1: Incomplete drying. If the precipitate is not dried to constant mass, residual water remains on or within the precipitate. This adds mass to the measured precipitate mass beyond the true dry mass, so the calculated moles of precipitate — and therefore the calculated analyte concentration — will be too high [1 identify + 1 mechanism]. Error 2: Coprecipitation. Impurity ions from solution can be adsorbed onto the surface of the growing precipitate crystals or physically trapped within the precipitate. This adds foreign mass to the precipitate that is not due to the analyte, inflating the precipitate mass above its true value and causing the calculated analyte amount to be too high [1 identify + 1 mechanism].

Marking notes. 1 mark per identified error (2 errors × 1) + 1 mark per mechanism explaining why the result is too high (2 × 1). Accept any two of: incomplete drying, coprecipitation, not subtracting filter paper mass, and failure to wash (soluble impurity ions trapped in wet precipitate). Each must be paired with a mechanistic explanation for the direction of effect.

Sample response (a). Ag⁺(aq) + Cl⁻(aq) → AgCl(s). [1]

Sample response (b). MM(AgCl) = 107.87 + 35.453 = 143.32 g mol⁻¹. n(AgCl) = 0.8614 ÷ 143.32 = 6.009 × 10⁻³ mol = n(Cl⁻) (1 : 1 ratio). [1] V(aliquot) = 25.00 mL = 0.02500 L. c(Cl⁻) in diluted solution = 6.009 × 10⁻³ ÷ 0.02500 = 0.2404 mol L⁻¹. [1+1]

Sample response (c). The 10.00 mL sample was diluted to 250.0 mL → dilution factor = 250.0 ÷ 10.00 = 25.0. c(Cl⁻)_brine = 0.2404 × 25.0 = 6.01 mol L⁻¹. [1] In g L⁻¹: 6.01 × 35.453 = 213 g L⁻¹. [1+1]

Sample response (d). The original brine is highly concentrated; diluting it produces a manageable precipitate mass (not too large or too small) and reduces the risk of incomplete precipitation or interference from other concentrated ions in the solution. [1]

Marking notes (a): correct ionic equation with state symbols [1]. (b): correct MM [1]; correct n(Cl⁻) [1]; correct c in diluted solution [1]. (c): correct dilution factor and application [1]; correct conversion to mol L⁻¹ in brine [1]; correct g L⁻¹ [1]. (d): any valid reason for dilution [1]. Accept slight rounding differences (±0.5%).

Sample response. Gravimetric analysis is a highly accurate technique for determining SO₄²⁻ in environmental water samples because its measurement — mass — is one of the most precisely quantifiable physical quantities. An analytical balance (±0.0001 g) introduces very little uncertainty. The near-complete insolubility of BaSO₄ (K_sp ≈ 1.1 × 10⁻¹⁰) ensures virtually all sulfate precipitates from solution, minimising systematic underestimation. In Australia, this technique is applied in contexts such as monitoring sulfate contamination in the Murray–Darling Basin irrigation water and testing mine drainage from the Pilbara region, where ppb-level accuracy is required. Several procedural steps are critical for maintaining accuracy. First, the precipitate must be dried to constant mass in an oven and cooled in a desiccator; failure to do so inflates the measured mass with residual water, giving a result that is too high. Second, only a moderate excess of BaCl₂ should be used: a slight excess drives complete precipitation (Le Chatelier effect), but a massive excess risks dissolving BaSO₄ as complex ions, reducing the precipitate mass and making the result too low. Third, the filter paper mass must be subtracted, and the precipitate must be washed with distilled water to remove co-precipitated impurities that would otherwise inflate the result. Despite its high intrinsic accuracy, gravimetric analysis is slow (hours per sample due to drying cycles) compared to volumetric titration (minutes). It is also poorly suited to analytes that do not form suitably insoluble precipitates. Modern alternatives such as ion chromatography or inductively coupled plasma (ICP) spectroscopy offer faster, multi-analyte analysis with comparable accuracy, making gravimetric analysis most useful when high-precision single-ion determination is required and where the slower turnaround is acceptable. In summary, gravimetric analysis achieves high accuracy for SO₄²⁻ by exploiting the near-total insolubility of BaSO₄ and precise mass measurement, provided the five-step procedure is followed rigorously to minimise drying, coprecipitation, and filter paper errors.

Marking criteria (7 marks):

• [1] Identifies at least two specific strengths of gravimetric analysis for SO₄²⁻ determination (e.g. high balance precision, near-complete precipitation due to low K_sp of BaSO₄).
• [1] Names a specific Australian environmental or industrial context correctly (Murray–Darling Basin water quality, Pilbara mine drainage, or other valid named Australian context).
• [1] Describes at least two procedural requirements that maintain accuracy, with mechanistic explanation of each (e.g. dry to constant mass prevents overestimation; moderate excess ensures complete precipitation).
• [1] Identifies at least one source of error that would make the result too high and one that would make it too low, with direction correctly stated.
• [1] Identifies at least one substantive limitation compared to an alternative technique (speed, multi-analyte limitation, or drying time).
• [1] Names an alternative quantitative technique (titration, ion chromatography, ICP) and contrasts it with gravimetric analysis on at least one criterion.
• [1] Reaches a clear evaluative judgement that integrates accuracy, procedural requirements, and limitations — does not simply list facts but arrives at a reasoned overall assessment.