HSC Exam Practice

Sample response. Absorbance (A) is a dimensionless measure of the fraction of incident light removed by a sample: A = log₁₀(I₀/I). Beer-Lambert law: A = εcl, where ε is molar absorptivity (L mol^-1 cm^-1), c is concentration (mol L^-1), and l is path length (cm).

Marking notes. 1 mark for defining absorbance with reference to I₀/I or log ratio; 1 mark for stating A = εcl correctly; 1 mark for identifying all three variables with correct units.

Sample response. The chemist prepares a series of standard solutions of known concentration [1]; measures the absorbance of each at a selected wavelength using a UV-Vis spectrophotometer with a cuvette of fixed path length, then plots absorbance vs concentration to produce a calibration curve [1]; measures the absorbance of the unknown sample under identical conditions and reads its concentration from the calibration curve or line equation [1].

Marking notes. 1 mark for preparing standards; 1 mark for constructing the calibration curve (absorbance vs concentration); 1 mark for measuring unknown absorbance and reading concentration from the curve.

Sample response. Each element has a unique set of characteristic wavelengths at which its atoms absorb light [1]. The hollow cathode lamp is constructed from (or coated with) the target element and emits only those characteristic wavelengths [1]. If a lamp for a different element were used, the free ground-state atoms of the target element in the flame would not absorb that light and no signal would be obtained [1].

Marking notes. 1 mark for the principle of element-specific characteristic wavelengths; 1 mark for explaining that the lamp emits only those wavelengths; 1 mark for the consequence of a mismatched lamp.

Sample response. (i) UV-Vis measures the absorption of UV/visible light by molecules or ions in solution; AAS measures the absorption of element-characteristic wavelengths by free ground-state atoms after atomisation. (ii) UV-Vis uses a broad-spectrum lamp with a monochromator; AAS uses a hollow cathode lamp specific to the target element. (iii) UV-Vis: the AWRI uses UV-Vis to analyse wine colourants and phenolics. AAS: NSW EPA uses AAS to monitor heavy metals (Pb, Cd, Cu) in river water or NSW Health uses AAS for blood lead level testing. [1 per criterion.]

Sample response. Limitation 1 — One element per run: AAS requires a different hollow cathode lamp for each element, so a sample containing multiple analytes (e.g. Pb, Cd, Cu together) must be re-run with each lamp change, increasing analysis time and cost [1 for limitation, 1 for effect/implication]. Limitation 2 — Matrix effects: other species in the sample can affect atomisation efficiency or absorb at nearby wavelengths, causing the measured absorbance to be higher or lower than it should be, leading to inaccurate concentrations [1 for limitation, 1 for effect/implication]. Accept also: instrument cost/complexity; cannot detect non-metals.

Sample response. Drawing a horizontal line from A = 0.217 to the calibration line and dropping vertically to the x-axis gives c ≈ 5.0 μg L^-1. [1 mark for correct graphical method; 1 mark for correct reading ±0.3 μg L^-1.]

Sample response. The calibration data shows that equal increments in concentration (2 μg L^-1) produce approximately equal increments in absorbance (≈0.087–0.088), confirming a directly proportional (linear) relationship [1]. This is consistent with Beer-Lambert law (A = εcl), which predicts linearity when ε and l are constant [1].

Sample response. A = 0.510 lies above the highest calibration standard (A = 0.433 at 10 μg L^-1), so determining the concentration would require extrapolation beyond the validated linear range [1]. Beyond this range, Beer-Lambert law may not hold (the relationship may curve), making the result unreliable [1]. The analyst should dilute the sample by a known factor until its absorbance falls within the calibrated range (e.g. a 1:2 dilution would give approximately A = 0.255), measure the absorbance of the diluted sample, read off the concentration from the calibration curve, and multiply by the dilution factor to obtain the original concentration [1].

Sample response. AAS was chosen for two main reasons. First, its detection limit for Cd is approximately 0.0003 mg L^-1 (0.3 μg L^-1), which is well within the concentration range monitored; UV-Vis has a detection limit for Cd of approximately 0.2 mg L^-1 (200 μg L^-1), making it incapable of detecting Cd at environmental concentrations [1 mark each for UV-Vis and AAS property with numbers; max 2]. Second, AAS is element-specific: the hollow cathode lamp targets only Cd-characteristic wavelengths, so co-occurring metals in river water do not interfere; UV-Vis colour reagents often complex multiple metals, reducing selectivity in complex matrices [1 mark each for UV-Vis and AAS selectivity; max 2].

Marking criteria.

• 1 mark — States Beer-Lambert law (A = εcl) and explains that it underpins quantitative spectroscopic analysis by linking absorbance to concentration.
• 1 mark — Describes the role of calibration curves in both UV-Vis and AAS: standards of known concentration are used to construct a curve, from which unknown concentrations are read.
• 1 mark — Names a real Australian UV-Vis application with correct analytical justification (e.g. AWRI wine colourant analysis: wine pigments strongly absorb visible wavelengths; UV-Vis measures absorbance, which is related to concentration via Beer-Lambert law).
• 1 mark — Names a real Australian AAS application with correct analytical justification (e.g. NSW EPA heavy-metal monitoring, or NSW Health blood lead testing).
• 1 mark — Compares the two techniques on sensitivity, explaining that AAS reaches much lower detection limits, making it essential for trace metal monitoring where concentrations are far below what UV-Vis can detect.
• 1 mark — Compares the two techniques on element specificity: AAS is element-specific (hollow cathode lamp + atomisation); UV-Vis is not inherently element-specific and can be used for organic or inorganic coloured species but lacks selectivity for individual metals in complex matrices.
• 1 mark — Identifies at least one limitation of each technique and acknowledges that neither is universally superior (e.g. AAS: one element per run; UV-Vis: unsuitable for trace metals; or UV-Vis: higher throughput but lower sensitivity).
• 1 mark — Reaches a justified overall evaluation: the choice of technique is task-dependent; UV-Vis is appropriate when the analyte is a strongly absorbing molecule or coloured complex in a clean matrix; AAS is required when very low concentrations of a specific metal element must be measured in a complex real-world matrix such as river water or blood.

Band 6 response indicators: precise chemical terminology throughout (molar absorptivity, atomisation, ground-state atoms, hollow cathode lamp, matrix effects, detection limit); explicit linkage of analytical properties to the two named Australian applications; nuanced final judgement that frames the choice as task-dependent and evidence-based rather than declaring one technique universally better.