Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 1

1 mark · Medium difficulty · Multiple Choice

Calculate the total number of ions present in 306 g of aluminium oxide, Al2O3.

Practise this question

Question

Multiple-choice question asking for the number of ions in 306 g of aluminium oxide, Al2O3. The given values are Avogadro constant = 6.02 x 10^23 mol^-1 and Molar mass of Al2O3 = 102 g mol^-1. Four options are provided: A 6.02 x 10^23, B 1.81 x 10^24, C 3.01 x 10^24, and D 9.03 x 10^24.
Question text

1 How many ions are present in 306 g of aluminium oxide, Al2O3?

[Avogadro constant = 6.02 q 1023 mol−1 Molar mass of Al O3 = 102 g mol−1 ]

A 6.02 q 1023

B 1.81 q 1024

C 3.01 q 1024

D 9.03 q 1024

(Total for Question 1 = 1 mark)

Mark scheme

Show the mark scheme Mark scheme indicating that D is the only correct answer (9.03 x 10^24), explaining why options A, B, and C are incorrect.

How to answer it

Calculating Total Ions in Ionic Compounds

What this question tests

This question assesses your understanding of the mole concept, molar mass calculations, Avogadro's constant, and crucially, stoichiometry involving ionic lattices where the number of constituent ions per formula unit must be accounted for.

Question 1

Full Worked Solution & Breakdown

✅ Correct Answer

D ( 9.03 × 10²⁴ )

Aluminium oxide consists of ions, and each formula unit yields a total of 5 individual ions (2 Al³⁺ and 3 O²⁻).

💡 Key Knowledge

  • Moles formula: Moles = Mass ÷ Molar Mass
  • Avogadro's constant: 1 mole contains 6.02 × 10²³ particles.
  • Formula analysis: Al₂O₃ splits into 2 Al³⁺ ions and 3 O²⁻ ions per formula unit. Total = 5 ions.

📐 Step-by-Step Calculation

  1. Calculate moles of Al₂O₃:
    Moles = 306 g ÷ 102 g mol⁻¹ = 3.0 mol
  2. Calculate formula units (molecules/entities):
    Units = 3.0 mol × 6.02 × 10²³ = 1.806 × 10²⁴
  3. Scale by total number of ions:
    Each Al₂O₃ gives 5 ions. Total ions = 1.806 × 10²⁴ × 5 = 9.03 × 10²⁴

🧠 Exam Technique

Multiple-choice questions often feature distractors corresponding to half-finished steps. Always read the stem carefully and underline key trigger words—here, the word was ions, not molecules!

❌ Common Errors & Distractor Analysis

  • Distractor A ( 6.02 × 10²³ ): This is the value for exactly 1 mole of molecules, ignoring both the mass given and the ion ratio.
  • Distractor B ( 1.81 × 10²⁴ ): This correctly calculates the number of formula units ( 3.0 × 6.02 × 10²³ ), but students often forget to multiply by the number of ions per formula unit (5).
  • Distractor C ( 3.01 × 10²⁴ ): This represents the total number of ions in just 1 mole of aluminium oxide ( 1 × 6.02 × 10²³ × 5 ), failing to scale up for the 306 g given.
Mark Scheme Note: (Total for Question 1 = 1 mark) — Awarded for selecting option D.

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.