OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 7

1 mark · Medium difficulty · Multiple Choice

Calculate the number of oxygen atoms in 120.2 g of SiO2.

Practise this question

Question

Multiple choice question 7 asking how many oxygen atoms are in 120.2 g of SiO2. Four options are provided: A (3.01 x 10^23), B (1.20 x 10^24), C (2.41 x 10^24), and D (3.61 x 10^24), along with an answer box and mark allocation of [1].
Question text

7 How many oxygen atoms are in 120.2 g of SiO2?

A 3.01 × 1023

B 1.20 × 1024

C 2.41 × 1024

D 3.61 × 1024

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 7 is C, worth 1 mark.

7 C 1 AO2.6

How to answer it

Calculating Number of Specific Atoms from Mass

What this question tests

This question assesses your mastery of the mole concept, molar mass calculations, Avogadro's constant, and scaling stoichiometric atom ratios within a given compound (OCR AS Level Chemistry, Module 2: Foundations in Chemistry).

Question 7 Multiple Choice Analysis

Correct Answer: C

✅ Correct Answer (C)

Option C ( 2.41 × 10²⁴ ) is correct. It correctly accounts for moles of molecules, scales by the 1:2 ratio of silicon dioxide to oxygen atoms, and multiplies by Avogadro's constant.

💡 Key Knowledge

  • Molar Mass (M): Calculated using atomic masses from the periodic table ( Si = 28.1 , O = 16.0 ).
  • Avogadro Constant (N_A): 6.022 × 10²³ mol⁻¹ represents the number of particles in one mole.
  • Atom Ratios: Each formula unit of SiO₂ contains 2 oxygen atoms.

🧠 Exam Technique

Multiple-choice stoichiometry questions are engineered to catch common algebraic slips. Always write down your formula rearrangement before plugging numbers into your calculator, and track units at every stage.

❌ Common Errors

  • Ignoring the 2:1 ratio: Forgetting to multiply by 2 for the oxygen atoms leads to option A ( 3.01 × 10²³ ).
  • Inverted calculations: Dividing mass by Avogadro's constant or multiplying by molar mass instead of dividing.

📐 Step-by-Step Calculation Guide

  1. Step 1: Calculate the molar mass of SiO₂
    M(SiO₂) = 28.1 + (2 × 16.0) = 60.1 g mol⁻¹
  2. Step 2: Find the number of moles of SiO₂ molecules
    Moles = Mass ÷ Molar Mass = 120.2 g ÷ 60.1 g mol⁻¹ = 2.00 mol
  3. Step 3: Determine the moles of oxygen atoms
    Since every 1 mole of SiO₂ contains 2 moles of O atoms:
    Moles of O = 2.00 mol × 2 = 4.00 mol
  4. Step 4: Convert moles of oxygen atoms to actual number of atoms
    Number of atoms = Moles × Avogadro constant (N_A)
    Number of O atoms = 4.00 mol × 6.022 × 10²³ mol⁻¹ = 2.4088 × 10²⁴ = 2.41 × 10²⁴ (to 3 significant figures).
Examiner Note: This item targeted AO2 (applying chemistry knowledge to familiar contexts). The distractor values correspond directly to failing to multiply by the stoichiometric subscript or inverting mathematical operations.

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.