Chemistry • Year 11 • Module 2 • Lesson 4
Gases & Molar Volume
Build HSC Band 5–6 technique on multi-step gas calculations, evaluating student proposals, and designing a procedure to identify an unknown gas by molar mass.
1. Data + evaluation — two students identify an unknown gas (Band 5–6)
9 marks Band 5–6
Scenario. A chemistry laboratory at a NSW school produces an unknown gas during a reaction. Two students each collect a sample and make the following measurements at SATP (25 °C, 100 kPa).
| Student A | Student B | |
|---|---|---|
| Volume of gas collected | 4.96 L | 9.92 L |
| Mass of gas collected | 8.80 g | 17.60 g |
| Conditions | SATP | SATP |
Illustrative data. Molar masses of common gases (g mol−1): CO2 = 44.01, SO2 = 64.06, N2 = 28.01, O2 = 32.00, CH4 = 16.04, C3H8 = 44.10.
Q1. Using both students’ data, determine the molar mass of the unknown gas and identify it. In your response you must:
- Calculate the number of moles in each student’s sample using n = V ÷ Vm, showing full working for both.
- Calculate the molar mass from each student’s data using MM = m ÷ n, and compare the two results.
- Identify the unknown gas and justify your identification by reference to the molar mass table.
- Explain why using two independent samples strengthens confidence in the identification.
- State one assumption you made and identify one source of error that could cause the calculated molar mass to differ from the true value.
2. Experimental design — identify an unknown gas using molar volume (Band 5–6)
8 marks Band 5–6
Research question. A chemist discovers an unlabelled cylinder of a pure gas in a storeroom. The gas is colourless and odourless. The chemist suspects it could be either carbon dioxide (CO2, MM = 44.01 g mol−1) or propane (C3H8, MM = 44.10 g mol−1). Because the molar masses are almost identical, a molar mass calculation alone may not distinguish between them reliably.
Equipment available: digital balance (±0.001 g), gas syringe (0–100 mL), barometer, thermometer, limewater (Ca(OH)2(aq)), distilled water, a lit splint, gas-washing bottles.
Q2. Design an investigation to identify the gas. In your response you must:
- State a hypothesis that includes the independent and dependent variables.
- Describe at least two separate tests you would perform (include a molar-volume molar-mass calculation AND at least one chemical identification test), with enough procedural detail for another student to follow.
- Predict the results for each test if the gas is CO2, and contrast with the predicted results if the gas is C3H8.
- Explain what result would falsify your hypothesis.
- State two limitations and one way to improve reliability.
Q1 — Sample Band 6 response (9 marks), annotated
Moles — Student A: Vm = 24.8 L mol−1 (SATP, 25 °C, 100 kPa). n = V ÷ Vm = 4.96 ÷ 24.8 = 0.200 mol [1]. MM = m ÷ n = 8.80 ÷ 0.200 = 44.0 g mol−1 [1].
Moles — Student B: n = 9.92 ÷ 24.8 = 0.400 mol [1]. MM = 17.60 ÷ 0.400 = 44.0 g mol−1 [1].
Comparison: Both students obtain MM = 44.0 g mol−1, consistent across a doubled sample, which increases confidence [1].
Identification: From the table, CO2 = 44.01 g mol−1 and C3H8 = 44.10 g mol−1. The calculated value (44.0 g mol−1) is consistent with CO2; however, the molar masses of CO2 and C3H8 are so close that this calculation alone cannot definitively distinguish them [1].
Why two samples strengthen confidence: Two independent samples of different volumes give the same molar mass, showing the result is reproducible and not the result of a random measurement error in a single trial [1].
Assumption: The gas behaves as an ideal gas at SATP; real gases deviate slightly from ideal behaviour, especially at high pressure or for polar molecules [1].
Source of error: If water vapour (from the reaction) is not removed before weighing, the measured mass will be higher than the true gas mass, making the calculated molar mass too large [1].
Marking criteria (9 marks): 1 = n(A) correct with working; 1 = MM(A) correct; 1 = n(B) correct; 1 = MM(B) correct; 1 = comparison of two results noting agreement; 1 = identification with reference to table; 1 = explanation of why two samples increase confidence; 1 = one valid assumption; 1 = one specific source of error.
Q2 — Sample Band 6 response (8 marks), annotated
Hypothesis: If the gas is CO2, it will have a calculated molar mass of approximately 44.01 g mol−1, will turn limewater milky, and will extinguish a lit splint. IV: gas identity (CO2 vs C3H8). DV: (i) calculated molar mass; (ii) effect on limewater; (iii) flammability [1].
Test 1 — Molar mass: (i) Record temperature and pressure to confirm SATP conditions. (ii) Collect exactly 50.0 mL (0.0500 L) of gas from the cylinder into a pre-weighed gas syringe. Seal the syringe. Weigh the syringe + gas and subtract the empty mass to find mass of gas. (iii) Calculate: n = 0.0500 ÷ 24.8 = 2.02 × 10−3 mol. MM = m ÷ n [1]. Predicted result: both CO2 and C3H8 give approximately 44 g mol−1, so this test alone is inconclusive for distinguishing them [1].
Test 2 — Limewater: Bubble a stream of the gas through 20 mL of limewater (Ca(OH)2(aq)) in a test tube for 30 s. Record whether the solution turns milky. CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) [1]. If CO2: limewater turns milky (white precipitate of CaCO3). If C3H8: limewater stays clear [1].
Test 3 — Flammability (optional confirmation): Release a small amount of gas from the syringe near a lit splint in a fume hood. If CO2: flame is extinguished. If C3H8: gas ignites with a yellow-orange flame [1].
Falsification: If the limewater stays clear AND the splint continues burning, the hypothesis (gas is CO2) would be falsified; the data would indicate C3H8 [1].
Limitations: (1) The molar-mass calculation alone cannot distinguish CO2 from C3H8 because their molar masses differ by only 0.09 g mol−1, within experimental uncertainty. (2) The limewater test cannot quantify the amount of CO2; trace amounts of CO2 contamination in C3H8 could give a false positive. Improvement: repeat each test three times (triplicate trials) and use a calibrated gas analyser (e.g. IR spectroscopy or a CO2 sensor) for a definitive chemical identification [1].
Marking criteria (8 marks): 1 = hypothesis with IV and DV; 1 = molar-mass test described with procedure; 1 = acknowledgement that MM test is inconclusive for these two gases; 1 = limewater test described with chemical equation; 1 = predicted contrasting results for CO2 vs C3H8; 1 = what would falsify the hypothesis; 1 = two limitations identified; 1 = one specific improvement.