4.1.1Organic Chemistry
Molar Volume of a Gas — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 33 of 42
Curriculum Equation Key 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)
Section 1 of 6Measuring Gas Volume
Two methods are used to collect and measure a gas.
Gas syringe
- A calibrated syringe is attached by a delivery tube and its plunger moves out as gas is produced.
Displacement of water
- The gas passes into an inverted measuring cylinder full of water and pushes water out of it.
Gas solubility
- Carbon dioxide is slightly soluble in water, so the syringe is more accurate for this gas.
Airtight apparatus
- All joints must be sealed, otherwise some gas escapes and the volume recorded is too low.
Section 2 of 6Method and Results
Examplecalcium carbonate reacting with excess hydrochloric acid, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).
1
Measure a fixed volume of excess dilute hydrochloric acid into a conical flask and weigh out a known mass of the solid carbonate.
2
Add the carbonate, immediately connect the delivery tube and start the timer if rate matters.
3
Wait until no more gas is formed and record the final volume shown on the syringe.
4
Repeat with several different masses and plot volume of gas against mass of carbonate.
5
Draw a single line of best fit ignoring anomalous points, then read off the volume for a chosen mass.
Section 3 of 6Calculating Molar Volume
Calculating Molar Volume
Moles of solid
- n = mass ÷ molar mass, using the molar mass of the carbonate.
Mole ratio
- One mole of carbonate gives one mole of carbon dioxide, so moles of gas equal moles of carbonate.
Volume units
- Convert cm³ to dm³ by dividing by 1000 before calculating.
Molar volume
- Molar volume = volume of gas ÷ moles of gas, which is about 24 dm³ mol⁻¹ at room temperature and pressure.
Section 4 of 6Sources of Error and Safety
Lost gas
- Gas escapes between adding the solid and sealing the apparatus, giving a volume that is too small.
Incomplete reaction
- If the acid is not in excess, the carbonate does not all react and the result is too low.
Acid hazards
- Dilute acids are irritants so eye protection is needed, and concentrated acids need gloves as well.
Syringe sticking
- A stiff plunger under-reads the volume, so lubricate it and tap it gently before reading.
Section 5 of 6Worked Example
Question
- 0.500 g of calcium carbonate (M = 100.1 g mol⁻¹) reacts completely with excess acid and produces 120 cm³ of carbon dioxide.
- Calculate the molar volume of the gas.
- Moles of CaCO₃ = 0.500 ÷ 100.1 = 0.004995 mol.
- The equation shows a 1 : 1 ratio, so moles of CO₂ = 0.004995 mol.
- Convert the volume120 cm³ ÷ 1000 = 0.120 dm³.
- Molar volume = 0.120 ÷ 0.004995 = 24.0 dm³ mol⁻¹.
Answer 24.0 dm³ mol⁻¹
Section 6 of 6Exam Checklist & Tips
4.1.1 – Key Points for the Exam
- Molar volume The volume occupied by one mole of any gas at a stated temperature and pressure.
- Gas syringe Calibrated glass syringe used to collect and measure the volume of a gas.
- Excess A reagent present in more than the amount needed, so the other reagent reacts completely.
- Line of best fit A single line or smooth curve drawn through plotted points that ignores anomalies.
Common Mistakes
- Forgetting to convert cm³ to dm³ before dividing, which gives an answer 1000 times too big.
- Using the mass of the metal instead of the molar mass of the whole carbonate to find moles.
- Joining points dot-to-dot rather than drawing a best-fit line, so anomalies distort the gradient.
- Using a gas syringe with leaks and not stating that the experiment assumes no gas is lost.
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