OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 2

1 mark · Medium difficulty · Multiple Choice

Calculate the mass of bromine required to react completely with 0.0200 mol of a diene to form a saturated compound.

Practise this question

Question

Question 2: 0.0200 mol of the compound below reacts with Br2(aq) to make a saturated compound. How many grams of Br2 are needed for this reaction? A skeletal formula of 5-chloropenta-1,3-diene is shown with a chlorine atom attached to a terminal CH2, followed by two conjugated carbon-carbon double bonds. Four options are given: A 3.20, B 6.39, C 7.99, D 12.80.
Question text

2 0.0200 mol of the compound below reacts with Br2(aq) to make a saturated compound.

How many grams of Br2 are needed for this reaction?

Cl

A 3.20

B 6.39

C 7.99

D 12.80

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme for Question 2 showing correct answer B with 1 mark.

2 B 1

How to answer it

Electrophilic Addition: Mass of Bromine Required

📋 What this question tests

This question assesses your ability to:

  • Interpret skeletal formulas of organic compounds and identify unsaturated functional groups (C=C double bonds).
  • Determine reacting stoichiometric ratios for electrophilic addition reactions leading to a fully saturated compound.
  • Carry out reacting mole-to-mass calculations using relative atomic/molecular masses ( m = n × M ).
Question 2 [1 Mark]

Stoichiometry of Addition to Dienes

Multiple Choice Analysis

✅ Correct Answer: B (6.39 g)

The organic molecule contains two C=C double bonds. Forming a saturated compound requires 2 moles of Br₂ for every 1 mole of the diene.

Mark Scheme: Award 1 mark for option B.

💡 Key Knowledge

  • Saturated: Contains only single C–C bonds (no C=C double or triple bonds).
  • Addition ratio: Each C=C double bond requires 1 molecule of Br₂ (adding 2 bromine atoms across the π-bond).
  • Molar mass of Br₂: From the OCR Periodic Table, Aᵣ(Br) = 79.9 , therefore M(Br₂) = 2 × 79.9 = 159.8 g mol⁻¹ .

📐 Step-by-Step Calculation

  1. Count the double bonds:
    Looking at the skeletal structure, the compound is 5-chloropenta-1,3-diene ( Cl–CH₂–CH=CH–CH=CH₂ ).
    Number of C=C double bonds = 2.
  2. Determine reacting mole ratio:
    1 diene + 2 Br₂ → saturated polyhaloalkane
    Reacting ratio = 1 : 2
  3. Calculate moles of Br₂ needed:
    n(Br₂) = 2 × n(diene) = 2 × 0.0200 mol = 0.0400 mol
  4. Calculate molar mass of Br₂:
    M(Br₂) = 2 × 79.9 = 159.8 g mol⁻¹
  5. Calculate required mass:
    mass = n × M = 0.0400 mol × 159.8 g mol⁻¹ = 6.392 g ≈ 6.39 g

❌ Common Errors & Distractor Breakdown

  • Selecting A (3.20 g): Missed the second C=C bond and calculated for a 1:1 ratio:
    0.0200 × 159.8 = 3.20 g .
  • Selecting C (7.99 g): Used atomic bromine ( Aᵣ = 79.9 ) incorrectly or assumed a 1:1 ratio with atomic mass.
  • Selecting D (12.80 g): Doubled the mole ratio twice (used a 1:4 ratio) by confusing 4 bromine atoms added with 4 Br₂ molecules:
    4 × 0.0200 × 159.8 = 12.78 ≈ 12.80 g .

🧠 Top-Grade Exam Technique

  • Count bonds carefully: Circle or tick each double bond on skeletal formulas immediately so none are overlooked.
  • Watch diatomic elements: Bromine exists as diatomic molecules ( Br₂ ). Always use 159.8 g mol⁻¹ , not 79.9 g mol⁻¹ , when calculating the mass of bromine reagent needed.
  • Quick mental estimate: 0.04 × 160 = 6.4 g . This instantly points to B without needing detailed manual long multiplication in Section A.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.