OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 7
1 mark · Medium difficulty · Multiple Choice
Determine the molecular formula of an alkane given the mass of water produced from the complete combustion of a known amount of the alkane.
Practise this questionQuestion
Question text
7 Complete combustion of 0.050 mol of an alkane produces 5.40 g of H2O.
What is the molecular formula of the alkane?
A C2H6
B C3H8
C C4H10
D C5H12
Your answer
[1]
Mark scheme
Show the mark scheme
7 D 1 AO1.2
How to answer it
Determining an Alkane Molecular Formula from Combustion
What this question tests
This question assesses your ability to apply quantitative chemistry concepts—specifically moles, molar mass, and stoichiometric reacting ratios—to determine the molecular formula of an alkane undergoing complete combustion (Assessment Objective AO1.2).
Complete Combustion and Molecular Formulas
✅ Correct Answer: D (C₅H₁₂)
Stoichiometric calculations show that 0.050 mol of the alkane produces 0.300 mol of H₂O, meaning each molecule of the alkane contains 12 hydrogen atoms, which corresponds exactly to pentane.
💡 Key Knowledge
- General Formula of Alkanes: CₙH₂ₙ₊₂
- Moles formula: Moles = Mass ÷ Molar Mass ( n = m / M )
- Combustion products: Complete combustion of any hydrocarbon yields carbon dioxide ( CO₂ ) and water ( H₂O ).
🧠 Exam Technique
For multiple-choice questions involving formulas, you can work backwards from the options by calculating the moles of water each option would produce, or calculate the mole ratio directly from the data given.
❌ Common Errors
- Forgetting that H₂O contains two hydrogen atoms per molecule when converting water moles to hydrogen atoms.
- Using the wrong molar mass for water (e.g., using 17 or 19 instead of 18.0 g mol⁻¹).
📐 Step-by-Step Calculation
- Find the moles of water produced:
Molar mass of H₂O = (2 × 1.0) + 16.0 = 18.0 g mol⁻¹.
Moles of H₂O = 5.40 g ÷ 18.0 g mol⁻¹ = 0.300 mol . - Determine the mole ratio of Alkane to Water:
Ratio = Moles of Alkane : Moles of H₂O
Ratio = 0.050 mol : 0.300 mol = 1 : 6 - Relate to the general formula ( CₙH₂ₙ₊₂ ):
The general combustion equation for an alkane is:
CₙH₂ₙ₊₂ + [(3n+1)/2] O₂ → n CO₂ + (n+1) H₂O
From the equation, 1 mole of alkane produces (n + 1) moles of H₂O .
Therefore, n + 1 = 6 , which means n = 5 . - Deduce the formula:
Substituting n = 5 into CₙH₂ₙ₊₂ gives C₅H₁₂ (Option D).
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, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.