OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify the formula of a compound that reacts with 8 mol of O2 for complete combustion from multiple choice options.

Practise this question

Question

Multiple choice question 15 stating that 1 mol of a compound reacts with 8 mol of O2 for complete combustion, asking for the formula of the compound. The options are A: C4H8, B: C4H9OH, C: C5H11OH, and D: C5H12.
Question text

15 1 mol of a compound reacts with 8 mol O2 for complete combustion.

What is the formula of the compound?

A C4H8

B C4H9OH

C C5H11OH

D C5H12

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 15 is D, worth 1 mark.

15 D 1

How to answer it

Complete Combustion Stoichiometry

What this question tests

This question assesses your understanding of stoichiometry in complete combustion reactions. You must be able to balance combustion equations for hydrocarbons and oxygen-containing organic compounds to relate moles of reactant to moles of oxygen required.

Question 15

Determining the Formula from Combustion Data

✅ Correct Answer

D: C₅H₁₂

Awarded 1 mark for selecting option D.

💡 Key Knowledge

  • Complete combustion of a hydrocarbon produces only CO₂ and H₂O .
  • General equation for combustion of CₓHᵧ :
    CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O
  • For oxygen-containing compounds ( CₓHᵧOH ), account for the oxygen already present in the molecule when balancing O₂ .

🧠 Exam Technique

Since this is a multiple-choice question, the fastest method is to write out or balance the combustion equation for each option until you find the one that requires exactly 8 mol of O₂ .

❌ Common Errors

  • Forgetting to divide the number of hydrogen atoms by 4 when calculating the O₂ balancing value.
  • Confusing alcohols with alkanes and miscounting oxygen atoms on the reactant side.

📐 Step-by-Step Calculation

Let's test option D (C₅H₁₂) using the combustion formula:

  1. Write the unbalanced equation:
    C₅H₁₂ + O₂ → CO₂ + H₂O
  2. Balance the carbon atoms:
    5 carbons give 5CO₂ .
  3. Balance the hydrogen atoms:
    12 hydrogens give 6H₂O ( 12 ÷ 2 = 6 ).
  4. Calculate total oxygen atoms needed on the product side:
    Products have (5 × 2) + (6 × 1) = 10 + 6 = 16 oxygen atoms.
  5. Find required moles of O₂ molecules:
    16 oxygen atoms require 16 ÷ 2 = 8 mol of O₂ . This matches the stem perfectly!

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.