Q: What does Gas Volume & Stoichiometry Investigations for H2 Chemistry Paper 4 cover? A: Everything needed to run leak-free gas-syringe, inverted burette, and balance-based investigations-apparatus setup, MMO routines, spreadsheet-ready PDO tables, and ACE commentary prompts anchored to MOE’s 2025 H2 Chemistry practical guidance.
TL;DR Paper 4 devotes 20 % of the H2 grade to P/MMO/PDO/ACE, and gas measurements frequently partner kinetics or redox contexts. Master leak checks, temperature logging, and spreadsheet conversions so volume–time or mass-loss data lead to defendable stoichiometric conclusions under exam timing.
1 | Where gas investigations sit in Paper 4
Section 4 of the 2025 H2 Chemistry syllabus lists gas collection as a recurring Paper 4 context testing Planning (apparatus choice), MMO (precise timing/measurement), PDO (structured tables/graphs), and ACE (diagnosing leaks or solubility losses).
Specimen Paper 4 tasks from Cambridge/SEAB typically pair gas collection with stoichiometric deductions (e.g., magnesium with acid, catalytic decomposition of hydrogen peroxide, or thermal decomposition of carbonates).
The 2026 SEAB specimen paper for 9476 Chemistry Paper 4 includes a gas-volume investigation that mirrors this workflow-2 h 30 min duration, Planning carried via pre-lab prompts, and MMO/PDO grids requiring precise volume logging.
Expect to justify the apparatus you choose, state how you record temperature/pressure, and describe how the data will be processed to obtain molar conclusions.
2 | Apparatus selection and setup discipline
Technique
Strengths
Limitations
Setup reminders
Gas syringe (0–100 cm³)
Direct volume readout, minimal processing
Plunger friction, limited to moderate gas volumes
Grease plunger lightly, clamp horizontally, run leak test before trials, align zero mark with plunger tip.
Inverted burette (water)
Handles >100 cm³, inexpensive, visible bubbles
Gas solubility in water, water vapour pressure must be corrected
Fill trough with same temperature water, eliminate bubbles, note atmospheric pressure for corrections, mark meniscus at eye level.
Mass loss (top-pan balance)
Robust for rapid reactions or soluble gases
Balance drift, spray losses, buoyancy variation with temperature
Zero balance with lid in place, protect pan with spill tray, log room pressure/temperature, avoid drafts and vibration.
Pre-run checklist
Inspect tubing, stoppers, and syringes for cracks; replace if they fail the leak test (hold pressure for 60 s with tap closed).
Calibrate gas syringes and burettes against known 10 cm³ aliquots to confirm graduations (±0.5 cm³ tolerance target).
Record ambient temperature and pressure to justify any molar volume assumptions (24.0 vs. 24.5 dm³ mol⁻¹).
Prepare reagents freshly, especially catalysts (e.g., MnO₂) or acids that release gases rapidly.
3 | MMO routines for trustworthy data
Leak test every setup. Charge the apparatus with a small measured volume of air, close taps, and watch for plunger drift or meniscus movement.
Synchronise timing. Start stopwatch/data logger exactly when reagents contact; for mass-loss setups, place flask on balance, tare, then add reagent quickly and replace lid.
Maintain mixing. Use a magnetic stirrer or consistent swirling to keep reaction rate uniform; note stirring speed if asked to repeat.
Log temperature. Record starting temperature and track if the reaction is exothermic; reference in ACE if significant drift (>2 °C) occurs.
Reset cleanly. Flush syringes/burettes with distilled water between runs, dry the balance pan, and re-zero to avoid residual reactants skewing data.
4 | Spreadsheet-ready PDO tables
Template columns: time (s), cumulative gas volume (cm³) or mass loss (g), corrected volume (cm³), moles of gas, theoretical moles, percentage difference, notes.
Corrections: adjust for water vapour pressure (if using water displacement) and convert to rtp using V_rtp = V_observed × (P_atm − P_vapour)/P_atm × 298/T.
Rate derivation: include columns for Δvolume/Δtime or use =SLOPE over selected data windows; graph volume vs. time and annotate gradient at early times for initial rate.
Uncertainty block: combine syringe tolerance (±0.5 cm³) or balance precision (±0.01 g) with timing uncertainty (±0.2 s) to report percentage uncertainty in the final stoichiometric result.
Documentation: note spreadsheet formulas (e.g., =AVERAGE(B3:B4), =C3/24000) so moderators understand how calculations were completed.
5 | Worked investigation templates
Hydrogen peroxide decomposition with MnO₂ (gas syringe)
Planning aim: Determine the volume of oxygen released per unit mass of catalyst to infer order dependence.
MMO focus: Start timing as soon as catalyst hits the peroxide solution; tap the syringe gently to release trapped bubbles.
PDO: Record readings every 10 s for 2 min; convert accumulated volume to moles and compare with theoretical stoichiometry 2H2O2→2H2O+O2.
ACE prompts: Discuss gas dissolution (oxygen in water), temperature rise (exothermic), and improvements such as using a thermostated bath or replacing water with saturated solution to reduce solubility.
Magnesium ribbon with dilute hydrochloric acid (mass loss)
Planning aim: Determine molar ratio and confirm purity of magnesium via mass-loss method.
MMO focus: Use pre-cut ribbons of known surface area, add acid swiftly, and replace the stopper to minimise spray.
PDO: Log mass every 5 s until steady; compute moles of hydrogen from total mass loss and compare with moles of magnesium present.
ACE prompts: Comment on acid mist losses, balance drift, buoyancy (heated air), and propose improvements such as absorption traps or performing reaction in a closed syringe system.
6 | ACE commentary scaffolds
Data quality diagnosis: Quote concordance between repeats (e.g., volumes within 0.8 cm³) and largest contributor to uncertainty (syringe tolerance vs. timing).
Systematic risks: Highlight leaks, gas solubility, delays in stopper placement, or catalyst clumping. Mention how temperature drift alters molar volume assumptions.
Improvements: Suggest performing blank corrections, using gas-tight syringes with PTFE plungers, applying water baths, or switching to pressure sensors/data loggers for higher fidelity.
Safety: Acknowledge hazards (oxygen enrichment, hydrogen flammability, acid exposure) and the controls you used (fume hood, splash guards, keeping ignition sources away).
Sample ACE paragraph:
Mean oxygen volume (52.6 cm³) differed by 0.9 cm³ across concordant trials, within the ±0.5 cm³ syringe tolerance. Temperature rose by 3 °C, so the true molar volume likely exceeded 24.0 dm³ mol⁻¹, explaining the 4 % deficit relative to theory. Repeating in a thermostated bath and pre-saturating the displacement water with oxygen would reduce solubility losses and tighten stoichiometric agreement.
7 | Quick readiness checklist
✅ Leak tested apparatus and documented tolerances.\
✅ Ambient temperature/pressure recorded for rtp corrections.\
✅ Spreadsheet template pre-loaded with conversion and uncertainty formulas.\
