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3.1.1Unit 3. Practical Skills in Chemistry I

Molar Volume of a Gas — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 3 of 10 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين المول والحسابات الكيميائية - Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو
Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.
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Section 1 of 6Collecting a Gas

Two methods allow the volume of a gas produced to be measured.

Gas syringe
  • Gas syringe A graduated syringe fitted to the flask collects the gas directly and gives volumes to about 1 cm³.
Displacement of water
  • Displacement of water Gas bubbles into an inverted measuring cylinder full of water, pushing water out so the volume can be read.
Solubility
  • Solubility Water displacement is unsuitable for gases that dissolve readily, for example sulfur dioxide or ammonia.
Airtight set-up
  • Airtight set-up All joints must be sealed, otherwise some gas escapes and the volume measured is too small.

Section 2 of 6Practical Method

ReactionNa₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g).

1
Measure a fixed volume of dilute hydrochloric acid, such as 25.0 cm³, into a conical flask.
2
Weigh a known mass of sodium carbonate and add it to the acid, then connect the gas syringe immediately.
3
Wait until the reaction is complete and the syringe plunger stops moving, then record the volume of carbon dioxide.
4
Repeat with several increasing masses of carbonate, making sure the acid is always in excess.
5
Plot volume of gas against mass of carbonate and draw one straight line of best fit, ignoring anomalous points.

Section 3 of 6Calculating the Molar Volume

The graph gives a reliable value of gas volume for a chosen mass.

Calculating the Molar Volume
Moles of solid
  • Moles of solid Use n = mass ÷ molar mass, with the mass read from the graph for the chosen point on the line.
Mole ratio
  • Mole ratio The equation shows 1 mol of Na₂CO₃ gives 1 mol of CO₂, so moles of gas equal moles of carbonate.
Convert units
  • Convert units Convert cm³ to dm³ by dividing by 1000 before dividing the volume by the moles of gas.
Molar volume
  • Molar volume At room temperature and pressure the value is about 24 dm³ mol⁻¹, so results should be close to this.

Section 4 of 6Hazards and Assumptions

Acids
  • Acids Dilute acids may be irritants, so eye protection is essential, and gloves for moderately concentrated solutions.
Gas loss
  • Gas loss Assume no gas escapes between adding the solid and connecting the syringe, though in practice a little is lost.
Completion
  • Completion Assume the reaction goes to completion, which is why the acid is used in excess.
Conditions
  • Conditions Gas volumes depend on temperature and pressure, so room conditions should be recorded.

Section 5 of 6Worked Example

Question

  • A sample of 1.00 g of a metal carbonate MCO₃ reacts with excess acid to give 285 cm³ of carbon dioxide at room temperature and pressure, where the molar volume is 24.0 dm³ mol⁻¹.
  • Identify M (Ar: C = 12.0, O = 16.0).

  1. Volume of CO₂ = 285 ÷ 1000 = 0.285 dm³.
  2. n(CO₂) = 0.285 ÷ 24.0 = 0.011875 mol.
  3. n(MCO₃) = n(CO₂) = 0.011875 mol, so Mr = 1.00 ÷ 0.011875 = 84.2.
  4. Ar of M = 84.2 − (12.0 + 3 × 16.0) = 84.2 − 60.0 = 24.2.
  5. The closest Group 2 metal is magnesium (Ar = 24.3).

Answer M is magnesium, so the carbonate is MgCO₃.

Section 6 of 6Exam Checklist & Tips

3.1.1 – Key Points for the Exam

  • Molar volume
    • The volume occupied by one mole of a gas under stated conditions;
      • about 24 dm³ at room temperature and pressure.
  • Gas syringe Graduated apparatus used to collect and measure the volume of a gas.
  • Line of best fit A straight line or smooth curve drawn to represent the trend of the data without joining every point.
  • Excess A reactant present in more than the amount needed, so the other reactant is used up completely.

Common Mistakes

  • Forgetting to convert cm³ to dm³ before calculating, which gives an answer 1000 times too large.
  • Using the whole carbonate formula mass as the metal mass instead of subtracting 60.0 for the CO₃ part.
  • Using a gas syringe with a leaking joint, which gives volumes that are too low.
  • Joining every point on the graph instead of drawing one line of best fit and excluding anomalies.

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