AQA A-Level Chemistry Paper 3, 2017: Question 28

1 mark · Medium difficulty · Multiple Choice

Calculate the mole fraction of carbon dioxide when a 385 cm³ sample at 100 kPa and 25 °C is mixed with argon.

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Question

Question 28 asks: A 385 cm³ sample of carbon dioxide at 100 kPa and 25 °C was mixed with 2.89 × 10⁻² mol of argon. The gas constant, R = 8.31 J K⁻¹ mol⁻¹. What is the mole fraction of carbon dioxide in the mixture? Options are A: 0.35, B: 0.46, C: 0.54, D: 0.65.
Question text

A 385 cm3 sample of carbon dioxide at 100 kPa and 25 °C was mixed with

2.89 × 10−2 mol of argon. The gas constant, R = 8.31 J K−1 mol−1

What is the mole fraction of carbon dioxide in the mixture?

[1 mark]

A 0.35

B 0.46

C 0.54

D 0.65

Mark scheme

Show the mark scheme Mark scheme indicates that for question 28, the correct answer is option A.

28 A

How to answer it

AQA A-Level Chemistry • Paper 1 • Physical Chemistry

Question 28: Mole Fraction & Ideal Gas Equation

What this question tests

This question assesses your ability to apply the ideal gas equation (pV = nRT) with correct SI unit conversions (pressure in Pa, volume in m³, temperature in K), calculate the total amount of gas in a mixture, and determine the mole fraction of a specific component.

Question Analysis & Solution

Multiple Choice (1 Mark)

✅ Correct Answer

A (0.35)

The carbon dioxide gas sample contains 0.0155 mol. When combined with 0.0289 mol of argon, the total moles equal 0.0444 mol, giving a mole fraction of 0.35.

💡 Key Knowledge

  • Ideal Gas Law: p V = n R T
  • Unit Conversions:
    • Pressure: 100 kPa = 100 × 10³ Pa
    • Volume: 385 cm³ = 385 × 10⁻⁶ m³
    • Temperature: 25 °C + 273 = 298 K
  • Mole Fraction (XA):
    XA = nA / ntotal

📐 Step-by-Step Calculation

  1. Convert all quantities to standard SI units:
    • p = 100 kPa = 100 000 Pa
    • V = 385 cm³ = 385 × 10⁻⁶ m³ = 3.85 × 10⁻⁴ m³
    • T = 25 °C = 25 + 273 = 298 K
    • R = 8.31 J K⁻¹ mol⁻¹
  2. Calculate the moles of carbon dioxide (nCO₂):

    nCO₂ = (p × V) / (R × T)
    nCO₂ = (100 000 × 385 × 10⁻⁶) / (8.31 × 298)
    nCO₂ = 38.5 / 2476.38 = 0.015547 mol (1.555 × 10⁻² mol)

  3. Calculate total moles in the mixture (ntotal):

    ntotal = nCO₂ + nAr
    ntotal = 0.015547 + 0.0289 = 0.044447 mol

  4. Calculate mole fraction of CO₂:

    Mole fraction (CO₂) = nCO₂ / ntotal
    Mole fraction = 0.015547 / 0.044447 = 0.3498 ≈ 0.35

❌ Common Distractor Traps

  • Option D (0.65): This is the mole fraction of argon ( 0.0289 / 0.0444 = 0.65 ). Always re-read which gas the question is asking for!
  • Option C (0.54): Dividing moles of CO₂ by moles of argon directly ( 0.0155 / 0.0289 = 0.54 ) instead of dividing by the total moles.
  • Unit Errors: Forgetting to convert cm³ to m³ (multiplying by 10⁻⁶) or kPa to Pa (multiplying by 10³).

🧠 Exam Technique & Examiner Tips

  • Sum of mole fractions equals 1: Notice that 0.35 + 0.65 = 1.00 . When you see paired complementary values in a multiple-choice question, double check whether you've calculated the fraction for the requested substance or the other one.
  • Pace your working: Multiple-choice questions that require full multi-step calculations still only award 1 mark. Write out your conversions cleanly in the margin to avoid silly keystroke errors.
Mark Scheme Note: 1 mark awarded solely for correct letter A. No partial marks for working in Section B multiple choice.

Topics

Physical Chemistry · 3.1.2 Amount of Substance · 3.1.10 Equilibrium Constant Kp

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.