OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 3

1 mark · Medium difficulty · Multiple Choice

Identify element X given the mass reacted with oxygen to form an oxide of formula X2O3.

Practise this question

Question

Multiple choice question 3: 0.80 g of element X is reacted with 0.40 g of O2 to form an oxide with the formula X2O3. What is the identity of element X? Options are A Aluminium, B Titanium, C Germanium, D Molybdenum. An answer box is provided at the bottom with mark allocation [1].
Question text

30.80 g of element X is reacted with 0.40 g of O2 to form an oxide with the formula X2O3.

What is the identity of element X?

A Aluminium

B Titanium

C Germanium

D Molybdenum

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 3 showing the correct answer as B, with 1 mark awarded.

3 B 1 2.2

How to answer it

Determining the Identity of Element X from Mass and Stoichiometry

What this question tests

This question assesses your ability to apply core mole calculations, interpret chemical formulae and reacting ratios, and use relative atomic masses (Ar) to identify an unknown element from experimental mass data.

Question 3 Multi-Choice Breakdown

Part 3: Calculating Molar Mass and Identifying Element X

Option B (Titanium)

✅ Correct Answer

The correct option is B (Titanium). Element X reacts to form an oxide with the formula X₂O₃ .

💡 Key Knowledge

  • Relating mass to moles using moles = mass / Ar (or Mr).
  • Using balancing coefficients in chemical equations or reacting mass ratios to find stoichiometric amounts.
  • Looking up periodic table values to match calculated molar masses to known elements.

🧠 Exam Technique

For multiple-choice questions involving calculations, do not guess. Work out the molar mass of X step-by-step and compare it directly with the relative atomic masses of the given options (Al, Ti, Ge, Mo).

❌ Common Errors

  • Treating oxygen as single atoms ( O ) instead of diatomic molecules ( O₂ ) when calculating initial moles.
  • Inverting the reacting ratio from the formula X₂O₃ .
  • Forgetting to account for the subscript 2 in X₂O₃ when calculating the molar mass of X.

📐 Step-by-Step Calculation

  1. Find moles of O₂ used:
    Mass of O₂ = 0.40 g
    Mr of O₂ = 16.0 × 2 = 32.0 g mol⁻¹
    Moles of O₂ = 0.40 / 32.0 = 0.0125 mol
  2. Use the stoichiometry of the reaction:
    The general equation for formation is: 4X + 3O₂ -> 2X₂O₃
    From the equation, 3 moles of O₂ react with 4 moles of X .
    Moles of X = Moles of O₂ × (4 / 3) = 0.0125 × (4 / 3) = 0.01667 mol
  3. Calculate the Relative Atomic Mass (Ar) of X:
    We know mass of X = 0.80 g and moles of X = 0.01667 mol.
    Ar = mass / moles = 0.80 / 0.01667 = 48.0 g mol⁻¹
  4. Identify the element:
    Checking the periodic table, the element with an Ar of approximately 48 is Titanium (Ti, Ar = 47.9).
Mark Scheme Note: 1 mark awarded for selecting B. Examiners note that top-tier candidates quickly set up the mole ratio without getting confused by the diatomic nature of oxygen.

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.