Edexcel A-Level Chemistry AS Paper 2, June 2017: Question 5

✅ Correct Answer

The volume occupied by one mole of a gas at a specified temperature and pressure (e.g., room temperature and pressure, standard temperature and pressure, or specified conditions).

❌ Common Errors

Students often forget to qualify the definition with "at a specified temperature and pressure". Gas volumes vary significantly with changing conditions, so mentioning the conditions is essential for the mark.

📐 Step-by-Step Calculation

1. Volume uncertainty:
   Apparatus uncertainty = ±0.5 cm³.
   Reading taken = 50 cm³.
   Percentage uncertainty = (0.5 / 50) × 100 = 1%
2. Mass uncertainty:
   Mass of gas = final mass - initial mass = 107.655 g - 107.563 g = 0.092 g .
   A balance reading has an uncertainty of ±0.0005 g. Because two readings are taken (empty syringe and syringe + gas), the total mass uncertainty is 0.0005 g × 2 = 0.001 g .
   Percentage uncertainty = (0.001 / 0.092) × 100 = 1.09% (or 1.1%).

🧠 Exam Technique & Traps

• The Balance Trap: Remember that electronic balances involve two separate weighings, meaning you must multiply the instrument tolerance by 2.
• Addition Trap: Examiners heavily penalise students who add absolute uncertainties together incorrectly instead of calculating the percentage uncertainty of the final derived value.

💡 Key Knowledge

When you double the volume of gas used to 100 cm³, both the measured volume and the mass of the gas double proportionally.

✅ Correct Answer

Volume uncertainty: Halves to 0.5% (because 0.5 cm³ / 100 cm³ × 100).

Mass uncertainty: Also halves, because the mass of gas collected doubles while the absolute balance uncertainty remains fixed at ±0.001 g.

📐 Step-by-Step Calculation

1. Find mass of gas:
   107.655 g - 107.563 g = 0.092 g
2. Use molar volume relationship:
   24 000 cm³ = 1 mol
   Moles of gas = 50 / 24 000 = 0.002083 mol
3. Calculate molar mass (M = m / n):
   Molar mass = 0.092 g / (50 / 24000 cm³) = 44.16 g mol⁻¹
4. Apply Significant Figures & Units:
   Round to 2 or 3 significant figures: 44 g mol⁻¹ or 44.2 g mol⁻¹ .

🧠 Top-Level Guidance

Alternative correct route: M = (mass / volume) × molar volume → (0.092 / 50) × 24000 = 44.16 g mol⁻¹ . Always check if the question requests specific significant figures; here, 2 or 3 SF is required along with explicit units ( g mol⁻¹ ).

✅ Correct Answer

How to spot: The plunger does not return to zero (or its original starting position) when released during the leak check in Step 2.

Effect on molar mass: The calculated molar mass will decrease because air entered the syringe. Air has an average molar mass of ~29 g mol⁻¹, which is lower than the pure gas (44 g mol⁻³ / CO₂ equivalent), lowering the average mass per mole.

❌ Common Errors

Students often state the plunger "moves" without specifying where it moves or failing to link the lower molar mass of air to the final calculated value.

💡 Key Knowledge

According to the Ideal Gas Law (or general gas behaviour), lowering the temperature at constant pressure causes a given mass of gas to occupy a smaller volume, or conversely, packs more molecules into a fixed volume.

✅ Correct Answer

1. The calculated molar mass would be greater.
2. Reason: At a lower temperature, more molecules/moles/mass are present in the same measured volume (density is greater), meaning you calculate an inflated mass per mole.

✅ Correct Answer

Water vapour would be present, turning the gas into a mixture, which would decrease or affect the overall measured molar mass.

❌ Common Errors

Do not state that "water reacts with the gas inside the syringe" unless specified, nor should you mention that wet gas is heavier. Focus purely on mixture composition and partial pressures of water vapour.