OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 16

1 mark · Medium difficulty · Multiple Choice

Identify the alkane that undergoes complete combustion to form 30 cm3 of carbon dioxide and 40 cm3 of water vapour under the same conditions.

Practise this question

Question

Multiple choice question 16 asks which alkane forms 30 cm3 of carbon dioxide and 40 cm3 of water vapour upon complete combustion under the same conditions. Four options are given: A butane, B ethane, C heptane, and D propane. A box is provided for the student's answer, worth 1 mark.
Question text

16 Complete combustion of an alkane forms 30 cm3 of carbon dioxide and 40 cm3 of water vapour,

under the same conditions of temperature and pressure.

Which alkane has undergone complete combustion?

A butane

B ethane

C heptane

D propane

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 16 is D, worth 1 mark.

16 D 1 AO2.6

How to answer it

Determining Alkane Formula from Combustion Volumes

What this question tests:

This question assesses your understanding of gas stoichiometry and molar volumes under constant conditions of temperature and pressure. You must link the stoichiometric volume ratios of carbon dioxide and water vapour produced during complete combustion directly to the molecular formula of the parent alkane.

Question 16 (Multiple Choice)

Solution & Breakdown

✅ Correct Answer: D (propane)

The correct option is D. Propane has the molecular formula C₃H₈ , which yields a 3 : 4 volume ratio of carbon dioxide to water vapour upon complete combustion.

💡 Key Knowledge

  • Avogadro's Law states that under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of moles.
  • Therefore, mole ratios = volume ratios for gaseous reactants and products.
  • General complete combustion equation for alkanes: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O

🧠 Exam Technique

Instead of working backward through complex balancing, use the given gas volumes directly as coefficients to set up your ratio: V(CO₂):V(H₂O) = 30 : 40 = 3 : 4 .

❌ Common Errors

Students often confuse the number of hydrogen atoms with the stoichiometric coefficient of water. Remember that water is H₂O , meaning the number of moles/volumes of water produced is always half the number of hydrogen atoms ( y/2 ) in the original alkane.

📐 Step-by-Step Calculation

  1. Find the volume ratio of products:
    Volume of CO₂ : Volume of H₂O = 30 cm³ : 40 cm³
  2. Simplify the ratio:
    Divide both sides by 10 to get 3 : 4 . This means for every 3 molecules of CO₂, 4 molecules of H₂O are formed.
  3. Relate coefficients to formula ( CₓHᵧ ):
    - The coefficient of CO₂ equals x , so x = 3 (Prop-).
    - The coefficient of H₂O equals y / 2 , so y / 2 = 4 , meaning y = 8 .
  4. Identify the alkane:
    An alkane with 3 carbons and 8 hydrogens is propane ( C₃H₈ ). Option D is correct.
Mark Scheme Award: 1 mark for selecting D (AO2.6 - Applying chemical knowledge to quantitative gas calculations).

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · 2.1 Atoms and reactions · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.