OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 2

1 mark · Medium difficulty · Multiple Choice

Identify which statement gives the numerical value of the Avogadro constant involving moles, mass, and electrons.

Practise this question

Question

Multiple choice question 2 asking which statement gives the numerical value of the Avogadro constant, with four options A to D involving moles of carbon-12, electrons lost by calcium, molecules of oxygen, and atoms in chlorine molecules, alongside an answer box.
Question text

2 Which statement gives the numerical value of the Avogadro constant?

A The number of moles in 12 g of carbon-12.

B The number of electrons lost by 20.05 g of calcium when it reacts with oxygen.

C The number of molecules in 16.0 g of oxygen.

D The number of atoms in 1 mole of chlorine molecules.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question number 2 with the correct answer B and 1 mark allocated.

2 B 1 1.2

How to answer it

Understanding the Avogadro Constant

What this question tests

This question assesses your precise understanding of the definition of the Avogadro constant (NA), mole calculations, and stoichiometry. You must be able to link mass to moles and connect moles of atoms/ions to the fundamental number of particles (electrons, atoms, or molecules).

Multiple Choice Question 2

Exam Breakdown & Working Out

✅ Correct Answer: B

Statement B correctly gives the numerical value of the Avogadro constant. Let's break down why:

  • Molar mass of calcium (Ca) = 40.08 g mol⁻¹.
  • Mass of calcium given = 20.05 g, which is exactly 0.5 mol of Ca atoms (20.05 / 40.08).
  • Calcium forms Ca²⁺ ions when reacting with oxygen, losing 2 electrons per atom.
  • Number of moles of electrons lost = 0.5 mol × 2 = 1.0 mol of electrons.
  • 1.0 mole of anything equals the Avogadro constant number of particles!

💡 Key Knowledge

The Avogadro constant is defined as the number of constituent particles (usually atoms or molecules) in one mole of a substance. Its numerical value is approximately 6.022 × 10²³ mol⁻¹.

  • 1 mol = 6.022 × 10²³ particles
  • Always check whether a species is monoatomic (like Ca), diatomic (like O₂ or Cl₂), or ionic.

🧠 Exam Technique

For multiple-choice mole questions, evaluate each option methodically by calculating the number of moles:

  • Always write down small rough calculations next to each option on your question paper.
  • Eliminate distractors that result in 2 moles, 0.5 moles, or incorrect particle types.

❌ Common Errors & Distractors

  • A: 12 g of carbon-12 is 1 mole of carbon-12 atoms, not the "number of moles" (which would just be 1).
  • C: 16.0 g of oxygen (O₂) is 0.5 moles of O₂ molecules, which contains 1 mole of oxygen atoms, but only 3.01 × 10²³ molecules.
  • D: 1 mole of chlorine molecules (Cl₂) contains 2 moles of chlorine atoms (2 × NA atoms).
Examiner Insight: This question discriminated well between students who memorise definitions blindly and those who can apply molar calculations and stoichiometry to electrons and atoms. Option D was a very common distractor chosen by students who forgot that chlorine is a diatomic molecule (Cl₂).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.