OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 13

1 mark · Medium difficulty · Multiple Choice

Calculate the partial pressure of oxygen in a gas mixture containing nitrogen and oxygen given the total pressure and the mole fraction of nitrogen.

Practise this question

Question

Multiple-choice question 13 asking for the partial pressure of O2 in a gas mixture of N2 and O2 with a total pressure of 1.42 atm and a mole fraction of N2 equal to 0.700. Four options are provided: A 0.211, B 0.426, C 0.493, and D 0.994, along with an answer box.
Question text

13 A mixture of N2 and O2 gases has a total pressure of 1.42 atm.

The mole fraction of N2 is 0.700.

What is the partial pressure, in atm, of O2 in the mixture?

A 0.211

B 0.426

C 0.493

D 0.994

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme for question 13 indicates that the correct answer is option B, worth 1 mark.

13 B 1 ALLOW 0.426 in the box

How to answer it

Calculating Partial Pressures and Mole Fractions

What this question tests

This question assesses your understanding of gaseous equilibria, specifically the relationship between total pressure, mole fractions, and partial pressures. You must demonstrate the ability to manipulate the fundamental partial pressure equation and recognize that the sum of all mole fractions in a mixture must equal 1.

Question Part 13

Multiple Choice Solution & Breakdown

✅ Correct Answer

The correct option is B (0.426).

💡 Key Knowledge

  • The sum of all mole fractions in a gas mixture is always equal to 1.00 .
  • Equation: Partial Pressure = Mole Fraction × Total Pressure .

🧠 Exam Technique

For multiple-choice calculations, always jot down your steps clearly on the exam paper margin before picking an option to avoid mental arithmetic traps.

❌ Common Errors

A frequent mistake is forgetting to find the mole fraction of O₂ first and mistakenly multiplying the given nitrogen mole fraction (0.700) directly by the total pressure.

📐 Step-by-Step Calculation

  1. Find the mole fraction of O₂:
    Since the mixture contains only N₂ and O₂, the sum of their mole fractions is 1.
    Mole fraction of O₂ = 1.000 - 0.700 = 0.300
  2. Calculate the partial pressure of O₂:
    Partial pressure = Mole fraction × Total pressure
    Partial pressure of O₂ = 0.300 × 1.42 atm = 0.426 atm
Mark Scheme Note: 1 mark awarded for selecting option B.

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 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.