Chemistry • Year 11 • Module 2 • Lesson 18
Working Scientifically — Practical Investigations
Build HSC Band 5–6 extended-response technique on evaluating experimental errors, designing controlled investigations, and synthesising precision vs validity arguments.
1. Data + scenario: evaluating a gravimetric analysis (Band 5–6)
8 marks Band 5–6
Scenario. A student determines the concentration of Ba2+ in a BaCl2 solution by gravimetric analysis. They add excess H2SO4(aq) to precipitate BaSO4(s), filter and weigh the precipitate. The true concentration of Ba2+ is 0.0500 mol/L. The table below shows the student’s results across three trials.
| Trial | Volume BaCl2(aq) used (mL) | Mass BaSO4 recorded (g) | Calculated [Ba2+] (mol/L) |
|---|---|---|---|
| 1 | 50.0 | 0.641 | 0.0550 |
| 2 | 50.0 | 0.639 | 0.0548 |
| 3 | 50.0 | 0.642 | 0.0550 |
M(BaSO4) = 233.4 g/mol. Illustrative data.
Q1. Analyse and evaluate the student’s gravimetric analysis results. In your response you must:
- Comment on the precision (reliability) of the three trials with reference to the data.
- Compare the student’s average calculated concentration with the true value and state whether the result is accurate.
- Identify the type of error present and justify your classification with reference to the direction and consistency of the results.
- Suggest one specific procedural error that could explain the systematic overestimate of [Ba2+], with a full mechanistic explanation of how it leads to this result.
- Propose two specific improvements to the procedure to improve both validity and reliability, explaining what each improvement achieves.
2. Experimental design — testing whether rinsing the burette matters (Band 5–6)
7 marks Band 5–6
Research question. A chemistry student claims: “It doesn’t matter if the burette is rinsed with titrant before filling — any water left in the burette has no effect on the titre because water doesn’t react with the analyte.” Design a controlled investigation to determine whether rinsing the burette with titrant before use has a measurable effect on titre values and calculated analyte concentration.
Constraints: You have 0.100 mol/L HCl(aq) (titrant), 25.0 mL aliquots of NaOH(aq) of unknown concentration, phenolphthalein indicator, burettes, pipettes, volumetric flasks, and an analytical balance. Design a two-condition experiment.
Q2. Design the investigation and present it in the format below.
- State a hypothesis including the independent variable, dependent variable, and predicted direction of effect.
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how the two conditions will differ and what measurements will be taken.
- Explain what result would falsify your hypothesis.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Precision (reliability): The three trials give calculated [Ba2+] values of 0.0550, 0.0548, and 0.0550 mol/L — a range of only 0.0002 mol/L, which is extremely small. This indicates the results are highly precise (reliable): the experiment is reproducible and random errors are small [1 mark — comment on precision with data reference].
Accuracy: The average calculated concentration = (0.0550 + 0.0548 + 0.0550) / 3 = 0.0549 mol/L. The true value is 0.0500 mol/L. The student’s results are ~9.8% above the true value (0.0049/0.0500 × 100%), so the results are not accurate despite being highly precise [1 mark — comparison with true value and accuracy judgement].
Error type and justification: The error is a systematic error. All three calculated values are consistently above the true value (0.0549 vs 0.0500 mol/L) by approximately the same amount. The error does not vary randomly around the true value; instead, it shifts all results in the same direction. Repeating the experiment (3 trials) has not reduced the discrepancy, confirming it is systematic rather than random [1 mark — classification with justification; 1 mark — evidence from consistent direction].
Procedural error: The most likely cause is incomplete drying of the BaSO4 precipitate before weighing. If the precipitate retains moisture, the recorded mass includes water as well as BaSO4. For example, if 0.641 g was recorded but the true dry mass was ~0.584 g, then n(BaSO4) = 0.641/233.4 = 2.75 × 10−3 mol (too high). Via the 1:1 stoichiometric ratio Ba2+ : BaSO4, the calculated n(Ba2+) is also too high, leading to c(Ba2+) = n/V being an overestimate. Because the same residual moisture is present in every trial (the same drying time was used each time), this error is systematic and consistent [1 mark — name the specific error; 1 mark — full mechanistic explanation].
Two improvements: (i) Dry the precipitate in an oven at 150°C and weigh it repeatedly until a constant mass is achieved (typically 2–3 successive weighings within 0.0005 g). This ensures all moisture is removed, eliminating the systematic overestimate and improving validity. (ii) Perform the experiment at least five times instead of three, so that any remaining random variation can be further reduced and a more reliable average can be calculated, improving reliability. Accept also: use an analytical balance (±0.0001 g) to reduce mass measurement uncertainty; filter using a pre-dried, pre-weighed sintered glass crucible to avoid contamination from filter paper. [1 mark per improvement with explanation; 2 marks total].
Marking criteria (8 marks): 1 = precision statement referencing data range; 1 = accuracy comparison with true value (average vs true, error percentage or absolute); 1 = correct classification as systematic error; 1 = evidence from consistent direction (same-direction bias across all trials); 1 = naming the specific procedural error (incomplete drying); 1 = full mechanistic explanation (moisture → mass too high → n(BaSO4) too high → c(Ba2+) overestimated); 1 = first improvement with explanation of what it achieves; 1 = second improvement with explanation of what it achieves.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If a burette is not rinsed with the titrant (0.100 mol/L HCl) before filling, then the residual water will dilute the HCl inside the burette, causing a larger titre compared to a rinsed burette, and leading to an overestimate of c(NaOH). Independent variable: whether the burette is rinsed with HCl before filling (rinsed vs unrinsed). Dependent variable: the average concordant titre (and subsequently the calculated c(NaOH)). [1 mark — testable hypothesis with IV and DV]
Controlled variables: Volume of NaOH aliquot (25.0 mL by pipette), concentration of HCl stock solution (0.100 mol/L), number of drops of indicator (2–3 drops), temperature of solutions, same student performing all titrations. [Accept any two]
Procedure: (1) Set up two burettes: Burette A is rinsed three times with ~5 mL aliquots of 0.100 mol/L HCl before filling; Burette B is filled directly with 0.100 mol/L HCl without rinsing, so ~1 mL of distilled water remains from the last wash. Fill both burettes to the 0.00 mL mark. (2) Using a calibrated 25.0 mL pipette, transfer a 25.0 mL aliquot of NaOH solution into a conical flask. Add 2–3 drops of phenolphthalein indicator. (3) Titrate the NaOH with HCl from Burette A, adding dropwise near the endpoint. Record the titre. Repeat to obtain three concordant titres (≤0.1 mL apart) for Condition A. (4) Repeat steps 2–3 using Burette B (unrinsed) for Condition B, obtaining three concordant titres. Calculate average concordant titres and c(NaOH) for each condition. Compare. [1 mark — four steps with both conditions and what is measured]
Falsification: If the concordant titres from the rinsed and unrinsed burettes are identical (within instrument uncertainty of ±0.05 mL), the hypothesis is falsified and the student’s claim would be supported. [1 mark]
Limitations: (1) The amount of residual water in the unrinsed burette is difficult to control precisely — different amounts of water remaining each time would change the degree of dilution and therefore the titre variability. (2) If the same student performs both conditions, they may unconsciously adjust their endpoint detection behaviour. A blinded procedure (student does not know which burette they are using) would reduce this bias. [1 mark per limitation; 2 marks total]
Improvement: Perform at least five titrations per condition (not three) to increase the pool of concordant values and reduce the influence of any individual random error on the average titre, improving reliability. [1 mark]
Marking criteria (7 marks): 1 = testable hypothesis naming IV (rinsed vs unrinsed) and DV (titre/c(NaOH)) with predicted direction; 1 = four procedural steps with both conditions differentiated and measurements specified; 1 = states what would falsify the hypothesis (identical titres within uncertainty); 1 = valid limitation 1 (e.g. uncontrolled water volume); 1 = valid limitation 2 (e.g. experimenter bias / endpoint subjectivity); 1 = one specific improvement for reliability; 1 = precise scientific terminology used throughout (systematic error, concordant, titre, c = n/V, dilution effect).