OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 4

1 mark · Medium difficulty · Multiple Choice

Identify which combination of reactant volumes for carbon monoxide and oxygen produces the largest volume of carbon dioxide at RTP based on the given stoichiometric equation.

Practise this question

Question

Multiple-choice question 4 showing the reaction 2CO(g) + O2(g) -> 2CO2(g). Four options A through D list different volume combinations of CO(g) and O2(g) measured at RTP, and asks which produces the largest volume of CO2(g).
Question text

4 Carbon monoxide reacts with oxygen to form carbon dioxide:

2CO(g) + O2(g) 2CO2(g)

Which volumes of CO(g) and O2(g) produce the largest volume of CO2(g)?

All gas volumes are measured at RTP.

A 1.00 dm3 CO and 4.00 dm3 O

B 2.00 dm3 CO and 3.00 dm3 O

C 3.00 dm3 CO and 2.00 dm3 O

D 4.00 dm3 CO and 1.00 dm3 O

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 4 indicating the correct answer is C, worth 1 mark.

4 C 1

How to answer it

Gas Volumes & Stoichiometry (OCR AS Level Chemistry)

What this question tests

This question assesses your understanding of reacting molar ratios in gas equations, Avogadro's Law (that equal volumes of gases under the same conditions contain the same number of moles), and how to identify the limiting reagent to find the maximum possible product yield.

Question 4

Determining Maximum Gas Product Volume

✅ Correct Answer

C ( 3.00 dm³ CO and 2.00 dm³ O₂ )

Awarded 1 mark for selecting C.

💡 Key Knowledge

  • Avogadro's Law: Under the same temperature and pressure, gas volume is directly proportional to moles.
  • Reacting Ratios: The balanced equation 2CO(g) + O₂(g) → 2CO₂(g) means 2 volumes of CO react with exactly 1 volume of O₂ to form 2 volumes of CO₂.

🧠 Exam Technique

Instead of converting volumes into moles and back, work directly with ratios! For every 1 unit of O₂ , you need twice that volume of CO . Check each option to see which provides enough reactants without wasting the limiting reagent, maximizing the final CO₂ volume.

❌ Common Errors

Students often incorrectly assume the option with the largest total starting volume of reactants gives the most product, forgetting that one reactant usually runs out first (limiting reagent).

📐 Step-by-Step Calculation Breakdown

  1. Examine the balanced equation: 2CO(g) + O₂(g) → 2CO₂(g) (Ratio of CO : O₂ : CO₂ is 2 : 1 : 2 ).
  2. Test Option C ( 3.00 dm³ CO and 2.00 dm³ O₂ ):
    • To react completely with 3.00 dm³ CO , you would need 3.00 / 2 = 1.50 dm³ of O₂ .
    • Since we have 2.00 dm³ of O₂ available, O₂ is in excess, and CO is the limiting reagent.
    • The volume of CO₂ produced equals the volume of the limiting reagent CO because of the 2:2 ratio, yielding 3.00 dm³ of CO₂ .
  3. Why other options yield less:
    • Option A gives only 1.00 dm³ CO₂ .
    • Option B gives 2.00 dm³ CO₂ (limited by CO ).
    • Option D gives 2.00 dm³ CO₂ (limited by O₂ , where 1.00 dm³ O₂ reacts with 2.00 dm³ CO , leaving 2.00 dm³ CO unreacted).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.