OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 9

1 mark · Medium difficulty · Multiple Choice

Calculate the value of the equilibrium constant Kp for the synthesis of ammonia given the equilibrium partial pressures and total pressure.

Practise this question

Question

Multiple choice question 9 presenting the reversible reaction N2(g) + 3H2(g) reversible 2NH3(g). Given the equilibrium partial pressure of N2 as 18.75 MPa, H2 as 2.50 MPa, and the total pressure as 25 MPa, candidates are asked to find the value of Kp in MPa^-2 from four options A (1.2 x 10^-4), B (0.048), C (0.075), and D (21).
Question text

9 The reversible reaction of nitrogen and hydrogen to form ammonia is shown below.

N2(g) + 3H2(g) 2NH3(g)

In the equilibrium mixture, the partial pressure of N2 is 18.75 MPa and the partial pressure of H2 is

2.50 MPa.

The total pressure is 25 MPa.

What is the value of K , in MPa−2?

p

A 1.2 × 10−4

B 0.048

C 0.075

D 21

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows question number 9 with the correct answer being B.

9 B 1 2.6

How to answer it

Calculating the Equilibrium Constant (Kp)

What this question tests

This question assesses your ability to manipulate gas pressures in a gaseous equilibrium mixture, deduce missing partial pressures using Dalton's Law of Partial Pressures, construct an expression for the equilibrium constant Kp , and perform a multi-step calculation with correct units and significant figures.

Question 9 — Multiple Choice Part

Equilibrium Partial Pressures and Kp

✅ Correct Answer

Option B: 0.048

Worth 1 mark in the OCR A-Level Chemistry Paper.

💡 Key Knowledge

  • Dalton's Law: Total pressure equals the sum of all individual partial pressures in the container: P(total) = P(N₂) + P(H₂) + P(NH₃) .
  • Kp Expression: For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) , the expression is Kp = P(NH₃)² / (P(N₂) × P(H₂)³) .

🧠 Exam Technique

When a total pressure is given alongside only some individual partial pressures, always check if you need to subtract to find the missing gas pressure before attempting to calculate Kp .

❌ Common Errors

  • Forgetting to cube the hydrogen partial pressure ( P(H₂)³ ) in the denominator due to the stoichiometric coefficient of 3.
  • Using the total pressure directly as the partial pressure for ammonia.

📐 Step-by-Step Calculation Guide

  1. Step 1: Find the partial pressure of ammonia, P(NH₃).
    Rearrange Dalton's Law: P(NH₃) = P(total) - P(N₂) - P(H₂)
    P(NH₃) = 25 - 18.75 - 2.50 = 3.75 MPa
  2. Step 2: Construct the Kp expression.
    Kp = P(NH₃)² / (P(N₂) × P(H₂)³)
  3. Step 3: Substitute values and evaluate.
    Kp = (3.75)² / (18.75 × (2.50)³)
    Kp = 14.0625 / (18.75 × 15.625)
    Kp = 14.0625 / 292.96875 = 0.04799...
  4. Step 4: Round to appropriate significant figures.
    Given data are given to 3 or 4 significant figures, so round to 2 or 3 sig fig giving 0.048 MPa⁻² (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), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.