OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 9

1 mark · Medium difficulty · Multiple Choice

Determine the molecular formula of a gaseous compound formed from the reaction of Cl2 and ClF3 based on their reacting volumes.

Practise this question

Question

Multiple choice question 9 asks for the molecular formula of a gaseous compound formed when 2.0 dm^3 of Cl2 gas reacts with 2.0 dm^3 of ClF3 gas to form 6.0 dm^3 of product, with 100% atom economy. Four options are provided: A, ClF; B, Cl2F3; C, Cl3F2; D, Cl3F3. There is an answer box provided.
Question text

92.0 dm3 of Cl gas reacts with 2.0 dm3 of Cl F gas to form 6.0 dm3 of a gaseous compound.

The reaction has 100% atom economy and all volumes are measured at the same temperature

and pressure.

What is the molecular formula of the compound formed?

A ClF

B Cl2F3

C Cl3F2

D Cl3F3

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer for question 9 is option A.

9 A 1 2.2

How to answer it

Determining Molecular Formula from Gas Volumes

OCR AS Level Chemistry • Amount of Substance

What this question tests

This question assesses your ability to relate gas volumes to stoichiometric balancing numbers using Avogadro's Law, apply the concept of 100% atom economy (conservation of all atoms into a single product), and deduce an unknown molecular formula.

Question 9 (Multiple Choice)

✅ Correct Answer: A

Option A (ClF) is the correct molecular formula.

Mark allocation: 1 mark

💡 Key Knowledge

  • Avogadro's Law: Under the same conditions of temperature and pressure, equal volumes of all gases contain the same number of molecules (and therefore moles).
  • Reacting volumes of gases are directly proportional to their balancing coefficients in a stoichiometric equation.
  • 100% Atom Economy: Means all atoms present in the reactants are combined completely to form a single product molecule, with no other by-products.

🧠 Exam Technique

Do not panic when dealing with unfamiliar interhalogen compounds. Treat the gas volumes as mole ratios immediately, set up a balanced equation placeholder, and use conservation of mass/atoms to find the formula.

❌ Common Errors

  • Assuming that subscripts in reactant molecules must simply add up directly without balancing the moles first.
  • Confusing atom economy with percentage yield. Remember 100% atom economy simply points to a combination reaction (A + B → C).

📐 Step-by-Step Calculation Guide

  1. Step 1: Simplify the gas volume ratio to find molar coefficients.
    Volume of Cl₂ : Volume of ClF₃ : Volume of gaseous compound = 2.0 dm³ : 2.0 dm³ : 6.0 dm³.
    Dividing through by 2.0 gives the mole ratio: 1 Cl₂ : 1 ClF₃ : 3 (gaseous compound) .
  2. Step 2: Write out the balanced equation framework.
    1 Cl₂(g) + 1 ClF₃(g) → 3 [Product]
  3. Step 3: Count total atoms on the reactant side (conservation of mass).
    - Total chlorine atoms (Cl) = 2 (from Cl₂) + 1 (from ClF₃) = 3 Cl atoms.
    - Total fluorine atoms (F) = 3 (from ClF₃) = 3 F atoms.
  4. Step 4: Distribute atoms equally across the 3 moles of product.
    Since 3 moles of the product contain 3 Cl atoms and 3 F atoms, 1 mole of the product must contain:
    • Cl atoms: 3 / 3 = 1
    • F atoms: 3 / 3 = 1
    Therefore, the molecular formula of the gaseous compound is ClF.

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), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.