Core Practical 1: Measure the Molar Volume of a Gas

Chemical Reaction

Reaction equation:
CaCO₃(s) + 2CH₃COOH(aq) → Ca(CH₃COO)₂(aq) + CO₂(g) + H₂O(l)

1 mole of calcium carbonate produces 1 mole of CO₂.

Volume of CO₂ collected is used to calculate molar volume.

Apparatus and Chemicals

Equipment

• Conical flask and bung with delivery tube
• Stand and clamp
• 100 cm³ and 50 cm³ measuring cylinders
• Water bath (for gas collection)
• Test tube and balance (2 d.p.)
• Pestle and mortar (optional)

Chemicals

• 1 mol dm⁻³ ethanoic acid (~30 cm³ per trial)
• Powdered calcium carbonate (~2 g total)

Safety Notes

• Wear eye protection at all times.
• Ethanoic acid is an irritant – avoid contact with skin and cuts.
• Ensure delivery tube is not blocked – pressure can build up.
• Handle glassware carefully to prevent breakage and spills.

Procedure

1. Add 30 cm³ of 1 mol dm⁻³ ethanoic acid to a boiling tube.
2. Set up the gas collection apparatus.
3. Place ~0.05 g of calcium carbonate into a dry test tube and weigh it accurately.
4. Remove the bung, tip in the carbonate, and immediately replace the bung.
5. Measure the volume of CO₂ gas collected.
6. Reweigh the empty test tube to determine the exact mass added.
7. Repeat with increasing masses (0.05 g → 0.40 g).
8. Record results and repeat inconsistent data.

Note: Do not exceed 0.40 g of calcium carbonate unless a larger collection vessel is used.

Sample Data

Analysis of Results

1. Plot a graph of mass of CaCO₃ (x-axis) against volume of CO₂ (y-axis).
2. Draw a line of best fit and extend to determine molar volume.
3. Calculate moles of CaCO₃ using moles = mass ÷ M_r.
4. Since 1 mol CaCO₃ → 1 mol CO₂, use this to find moles of CO₂.
5. Calculate molar volume = volume of CO₂ ÷ moles CO₂ (convert to dm³).

Sample Calculation:
0.40 g CaCO₃: moles = 0.40 ÷ 100.1 = 0.003996 mol.
Volume collected ≈ 96 cm³ = 0.096 dm³.
Molar volume = 0.096 ÷ 0.003996 ≈ 24.0 dm³ mol⁻¹.

Common Errors and Improvements

• Main error: gas loss when tipping carbonate and replacing bung.
• Improvement: use flask-in-flask setup to ensure sealed mixing.

Extension Questions

• Why weigh test tube before and after?
  It accounts for substance left inside.
• Why replace bung quickly?
  To prevent gas loss.
• How confirm acid in excess?
  Calculate moles. 30 cm³ of 1 mol dm⁻³ = 0.030 mol. 0.40 g CaCO₃ = 0.004 mol. Ratio required = 2:1, so 0.030 > 0.008 → acid is in excess.

Matt’s exam tip

Always plot graphs to use at least half the available space. Keep scales simple and check if your line should pass through the origin.