OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 9

1 mark · Medium difficulty · Multiple Choice

Calculate the total number of ions in 4.00 mol of magnesium chloride, MgCl2.

Practise this question

Question

Multiple choice question 9 asks for the number of ions in 4.00 mol of magnesium chloride, MgCl2, with four options A (1.81 x 10^24), B (2.41 x 10^24), C (4.82 x 10^24), and D (7.22 x 10^24).
Question text

9 What is the number of ions in 4.00 mol of magnesium chloride, MgCl2?

A 1.81 × 1024

B 2.41 × 1024

C 4.82 × 1024

D 7.22 × 1024

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 9 is D, worth 1 mark.

9 D 1 AO2.6

How to answer it

Calculating the Total Number of Ions in an Ionic Lattice

What this question tests: This question tests your ability to apply the Avogadro constant, understand formula unit stoichiometry, and distinguish between the number of molecules/formula units versus the total number of individual ions present in an ionic substance.
Question 9 — Multiple Choice [1 Mark]

Exam Breakdown

✅ Correct Answer: D

Option D ( 7.22 × 10²⁴ ) is the correct choice because it accounts for both the moles of formula units and the total number of ions per formula unit of magnesium chloride.

💡 Key Knowledge

  • Formula Breakdown: 1 mole of MgCl₂ dissociates to form 1 mole of Mg²⁺ ions and 2 moles of Cl⁻ ions.
  • Total Ions Ratio: Each individual formula unit yields 3 ions in total (1 magnesium + 2 chloride).
  • Avogadro Constant (N_A): Approximately 6.022 × 10²³ mol⁻¹ .

🧠 Exam Technique

Always underline key command words in the stem—here, the word is ions, not molecules or formula units. Watch out for traps where the examiner provides a distractor calculated using only the moles of the compound.

❌ Common Errors

  • Distractor A ( 1.81 × 10²⁴ ): Results from forgetting to multiply by the 3 ions per formula unit, or incorrectly dividing.
  • Distractor C ( 4.82 × 10²⁴ ): Often happens if students mistakenly multiply by 2 (only counting chloride ions) instead of 3 total ions.

📐 Step-by-Step Calculation

  1. Step 1: Identify the number of ions per formula unit.
    Formula = MgCl₂ → Mg²⁺ + 2Cl⁻ = 3 ions total per formula unit.
  2. Step 2: Find the total moles of ions.
    Total moles = Moles of MgCl₂ × 3 = 4.00 mol × 3 = 12.0 mol of ions .
  3. Step 3: Convert moles of ions to actual number of particles using Avogadro's constant. .
    Number of ions = 12.0 mol × 6.022 × 10²³ mol⁻¹ = 7.2264 × 10²⁴ .
  4. Step 4: Round to appropriate significant figures.
    The data in the question ( 4.00 mol ) is given to 3 significant figures, giving 7.22 × 10²⁴ .
Mark Scheme Note: Correct option is D [1 mark]. AO2.6 (Applying numerical skills to chemical contexts).

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 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.