H2 Chemistry Gas Collection Planning: Displacement of Water and CO2 Volume

If $0.00250 \text{ mol}$ of carbonate is the limiting reagent, then $0.00250 \text{ mol}$ of CO2 is produced.

At room conditions, using $24.0 \text{ dm}^3 \text{ mol}^{-1}$:

$$V = 0.00250 \times 24.0 = 0.0600 \text{ dm}^3 = 60.0 \text{ cm}^3$$

That volume fits a 100 cm3 gas syringe or measuring cylinder. If the predicted volume is only 3 cm3, percentage uncertainty will be high. If it is 180 cm3, the collection vessel is too small.

3 | Limiting reagent check

For planning, show that the reagent producing the gas is limiting and the other reagent is in excess.

This prevents a common planning error: predicting CO2 volume from the wrong reagent.

4 | Displacement of water precautions

Useful planning precautions:

• Fill the delivery tube with water first if the question is about avoiding lost gas at the start.
• Check the bung and tubing for leaks before adding acid.
• Add acid quickly and replace the bung immediately.
• Keep the end of the delivery tube below the water surface.
• Ensure the measuring cylinder is completely filled with water before inverting.
• Keep temperature constant, or record it for molar-volume correction.
• Use saturated salt solution or a gas syringe if dissolving of the gas is a serious limitation.

For CO2, mention solubility in water as a systematic loss. The collected volume may be smaller than the true volume produced.

5 | Confirming carbon dioxide

Carbon dioxide turns limewater milky because calcium carbonate forms.

Exam phrasing:

Bubble a sample of the gas through limewater. A white precipitate or milky suspension confirms carbon dioxide.

Do not rely only on "gas puts out a flame" for CO2 identification. That observation is less specific.

6 | Planning-answer structure

Use this order when a question asks you to plan a gas-volume investigation:

1. State the independent variable, dependent variable, and controlled variables.
2. Predict gas volume from stoichiometry.
3. Choose apparatus with suitable range and precision.
4. Describe the collection method and leak check.
5. State timing and endpoint, such as volume recorded when bubbling stops.
6. Explain how repeated readings and mean volume will be used.
7. Identify the main limitation and improvement.

One concise ACE sentence:

CO2 dissolves in water, so the measured gas volume is likely lower than the true volume; using a gas syringe or collecting over saturated solution would reduce this systematic loss.

7 | Related guides

• Gas volume and stoichiometry investigations for H2 Chemistry Paper 4
• H2 Chemistry volumetric practical deep dive
• H2 Chemistry experiments hub

Sources

1. SEAB - H2 Chemistry (Syllabus 9476) GCE A-Level 2026
2. SEAB - H2 Chemistry Specimen Paper 4 (9476/04), first exam 2026

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