OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 3

1 mark · Medium difficulty · Multiple Choice

Calculate the total number of hydrogen atoms present in 6.8 g of ammonia.

Practise this question

Question

Question 3 asks: 'What is the number of hydrogen atoms in 6.8 g NH3?' with four multiple choice options: A is 2.408 times 10 to the power of 23, B is 3.612 times 10 to the power of 23, C is 7.224 times 10 to the power of 23, and D is 1.806 times 10 to the power of 24. A response box for 'Your answer' is shown on the right with a mark value of 1.
Question text

3 What is the number of hydrogen atoms in 6.8 g NH3?

A 2.408 × 1023

B 3.612 × 1023

C 7.224 × 1023

D 1.806 × 1024

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table row for question 3 indicating the correct answer is C, awarding 1 mark.

3 C 1

How to answer it

Calculating Number of Hydrogen Atoms in Ammonia (NH₃)

📌 What this question tests

This question assesses your ability to interconvert mass, amount of substance (moles), and the actual number of particles using the Avogadro constant ( NA = 6.02 × 10²³ mol⁻¹ ). It also tests stoichiometry within a molecular formula: recognizing that one molecule of ammonia contains three hydrogen atoms.

Question 3 • Multiple Choice [1 Mark]

Finding the Total Number of Hydrogen Atoms

"What is the number of hydrogen atoms in 6.8 g NH₃?"

✅ Correct Answer

Option C: 7.224 × 10²³

Awarded for the correct selection from options A, B, C, and D.

💡 Key Knowledge

  • Molar mass of NH₃: M = 14.0 + (3 × 1.0) = 17.0 g mol⁻¹
  • Moles formula: n = m / M
  • Avogadro's constant: NA = 6.02 × 10²³ mol⁻¹
  • Atoms per molecule: 1 molecule of NH₃ contains 3 H atoms and 1 N atom.

📐 Step-by-Step Calculation

  1. Calculate the amount (in moles) of NH₃:
    n(NH₃) = mass / Molar mass = 6.8 g / 17.0 g mol⁻¹ = 0.40 mol
  2. Calculate the total number of NH₃ molecules:
    Number of NH₃ molecules = 0.40 mol × (6.02 × 10²³ mol⁻¹) = 2.408 × 10²³ molecules
  3. Determine the number of hydrogen atoms:
    Each NH₃ molecule has 3 hydrogen atoms.
    Total H atoms = 3 × (2.408 × 10²³) = 7.224 × 10²³ atoms

Alternative method: Moles of H atoms = 0.40 mol × 3 = 1.20 mol . Total H atoms = 1.20 mol × 6.02 × 10²³ mol⁻¹ = 7.224 × 10²³ .

🧠 Exam Technique

  • Read the particle required: Always underline whether the question asks for molecules, ions, or a specific type of atom.
  • Estimate to verify: 6.8 / 17 is roughly 0.4 . 0.4 × 3 = 1.2 . 1.2 × 6 × 10²³ ≈ 7.2 × 10²³ . You can solve this in seconds without complex typing!
  • Watch the exponents: Ensure you check powers of 10 carefully when selecting your final letter.

❌ Distractor Breakdown & Common Traps

  • Distractor A (2.408 × 10²³): The most common trap! This is the number of NH₃ molecules. Candidates forget to multiply by 3 for hydrogen atoms.
  • Distractor B (3.612 × 10²³): Arises from calculation slips, such as using an incorrect molar mass or partial factors.
  • Distractor D (1.806 × 10²⁴): Calculated by taking 3 × 6.02 × 10²³ , omitting the mole calculation ( 6.8 / 17 ) altogether.
Examiner Insight: Multiple choice questions on stoichiometry frequently include the intermediate value (e.g., number of molecules) as one of the options. Always perform a quick sanity check: since each molecule contains multiple atoms, the total number of atoms must be greater than the number of molecules.

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), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.