Edexcel A-Level Chemistry Paper 2, November 2021: Question 1

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Question

Answer Mark

Number

1 The only correct answer is D (3.6 x 1023) (1)

A is not correct because the number of moles of (NH4)2SO4 has been divided by 3, rather than multiplied by 3

B is not correct because it is the number of SO 2−ions

C is not correct because it is the number of NH + ions

How to answer it

Calculating Total Number of Ions in Ionic Compounds

What this question tests

This question assesses core quantitative chemistry skills: calculating moles from mass and molar mass, applying the Avogadro constant to find discrete particles, and breaking down ionic lattice stoichiometry into individual constituent ions (cations and anions).

Question Breakdown

Multiple Choice Question (1 Mark)

✅ Correct Answer

D ( 3.6 × 10²³ )

Ammonium sulfate dissociates to give a total of 3 ions per formula unit (2 ammonium ions and 1 sulfate ion). Multiplying the moles of formula units by Avogadro's constant and by 3 yields the correct total number of ions.

💡 Key Knowledge

Moles formula: moles = mass ÷ molar mass ( n = m / M )
Avogadro constant ( L or N_A ): 6.0 × 10²³ mol⁻¹
Formula breakdown: (NH₄)₂SO₄ contains 2 × NH₄⁺ and 1 × SO₄²⁻ . Total = 3 ions.

🧠 Exam Technique

In multiple-choice questions involving ions or atoms within a molecule, distractors are specifically designed using partial calculations (e.g., forgetting to multiply by the total number of ions, or only counting one type of ion). Always read the question carefully to check whether it asks for molecules, formula units, or total ions.

Step-by-Step Calculation

How to arrive at option D

Step 1: Calculate the moles of ammonium sulfate

Use the given mass and molar mass:

Moles = 26.4 g ÷ 132 g mol⁻¹ = 0.200 mol of (NH₄)₂SO₄

Step 2: Determine the total moles of ions

Consider the dissociation equation:

(NH₄)₂SO₄ (aq) → 2NH₄⁺ (aq) + SO₄²⁻ (aq)

1 mole of (NH₄)₂SO₄ produces 3 moles of ions in total.

Total moles of ions = 0.200 mol × 3 = 0.600 mol

Step 3: Convert moles of ions to number of particles

Multiply by the Avogadro constant:

Number of ions = 0.600 mol × 6.0 × 10²³ mol⁻¹ = 3.6 × 10²³

Examiner Insights & Common Errors

Why students lose marks on similar questions

❌ Distractor A ( 4.0 × 10²² )

Occurs if a student incorrectly divides the moles of the compound by 3 instead of multiplying by 3 ( 0.200 ÷ 3 × 6.0 × 10²³ ).

❌ Distractor B ( 1.2 × 10²³ )

Represents just the number of sulfate ions ( SO₄²⁻ ). The student calculated 0.200 × 6.0 × 10²³ , failing to account for the ammonium ions.

❌ Distractor C ( 2.4 × 10²³ )

Represents just the number of ammonium ions ( NH₄⁺ ). The student multiplied by 2 instead of 3, forgetting to include the single sulfate ion in the total count.

Topics

Physical Chemistry · Topic 5: Formulae, Equations and Amounts of Substance