OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 9

1 mark · Medium difficulty · Multiple Choice

Calculate the partial pressure of SO3 given the equilibrium moles of SO2, O2, and SO3 and the total pressure.

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Question

Multiple choice question 9 asks for the partial pressure of SO3 in an equilibrium mixture of 2.4 mol SO2, 1.2 mol O2, and 0.4 mol SO3 at a total pressure of 250 atm, for the reaction 2SO2(g) + O2(g) reversible arrow 2SO3(g). Four multiple choice options are provided: A, 15 atm; B, 25 atm; C, 100 atm; D, 200 atm.
Question text

9 The reversible reaction of sulfur dioxide and oxygen to form sulfur trioxide is shown below.

2SO2(g) + O2(g) 2SO3(g)

An equilibrium mixture contains 2.4 mol SO2, 1.2 mol O2 and 0.4 mol SO3.

The total pressure is 250 atm.

What is the partial pressure of SO3?

A 15 atm

B 25 atm

C 100 atm

D 200 atm

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 9 showing the correct answer as B, worth 1 mark.

9 B 1 AO1.2

How to answer it

Calculating Partial Pressure at Equilibrium

What this question tests

This question assesses your understanding of gaseous equilibria, specifically how to calculate the mole fraction of an individual gas and use it alongside the total pressure to determine its partial pressure.

Question 9 Breakdown

Equilibrium Partial Pressure Calculation

✅ Correct Answer

B (25 atm)

The correct option is B because the mole fraction of sulfur trioxide is 0.10, which when multiplied by the total pressure of 250 atm gives 25 atm.

💡 Key Knowledge

  • Mole Fraction: Moles of gas / Total moles of all gases in the mixture.
  • Partial Pressure: Mole fraction × Total pressure.
  • The sum of all mole fractions in a mixture must always equal 1.

🧠 Exam Technique

Always start multiple-choice calculation questions by writing down the formulae you need. Ensure you calculate the total number of moles accurately before finding individual fractions.

❌ Common Errors

  • Forgetting to include all species when finding total moles.
  • Dividing by the moles of a single reactant instead of the sum of all moles.
  • Confusing mole fractions with stoichiometric coefficients from the balanced equation.

📐 Step-by-Step Calculation Guide

  1. Find the total number of moles at equilibrium:
    Total moles = moles(SO₂) + moles(O₂) + moles(SO₃)
    Total moles = 2.4 + 1.2 + 0.4 = 4.0 mol
  2. Calculate the mole fraction of SO₃:
    Mole fraction of SO₃ = Moles of SO₃ / Total moles
    Mole fraction of SO₃ = 0.4 / 4.0 = 0.10
  3. Calculate the partial pressure of SO₃:
    Partial pressure = Mole fraction × Total pressure
    Partial pressure = 0.10 × 250 atm = 25 atm
Examiner Insight: Students often incorrectly use the stoichiometric coefficients from the balanced equation (e.g., dividing by 2 or 3) instead of summing the actual equilibrium moles given in the stem. Always check that your mole fraction denominator accounts for every mole present in the equilibrium vessel.

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.