OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 3

1 mark · Medium difficulty · Multiple Choice

Identify element X given the mass of element X and the mass of its oxide X2O3 formed by reaction with oxygen

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Question

Multiple choice question 3 asking to identify element X. 0.541 g of element X reacts with oxygen to form 0.790 g of oxide X2O3. Four options are given: A Al, B Cr, C Ga, D Sc.
Question text

3 0.541 g of an element X is reacted with oxygen to form 0.790 g of the oxide X2O3.

What is the element X?

A Al

B Cr

C Ga

D Sc

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer is B.

3 B 1 2.2

How to answer it

Determining Element X from Oxide Mass

What this question tests

This question assesses your ability to apply quantitative chemistry concepts including mass conservation, reacting mole ratios, and molar mass calculations. You must use experimental mass data for a reacting element and its resulting metal oxide formula (X₂O₃) to deduce the relative atomic mass (Ar) and successfully identify element X from the periodic table.

Question 3 (Multiple Choice)

Full Worked Solution & Examiner Breakdown

✅ Correct Answer

B (Cr)

Marks: 1 / 1

💡 Key Knowledge

  • The formula X₂O₃ tells us that 2 moles of element X combine with 3 moles of oxygen atoms (O).
  • Conservation of mass allows us to find the mass of oxygen reacted by subtracting the initial mass of element X from the total mass of the oxide.

🧠 Exam Technique

When given multiple-choice options with similar atomic masses (like Al = 27.0, Cr = 52.0, Ga = 69.7, Sc = 45.0), avoid guessing. Perform a quick, structured calculation using moles to find the exact molar mass of X, then check your periodic table.

❌ Common Errors

  • Stoichiometry trap: Forgetting to account for the subscript '2' in X₂O₃ when calculating moles of X or determining molar mass.
  • Mass confusion: Using the mass of the oxide directly instead of finding the isolated mass of oxygen via subtraction ( 0.790 g - 0.541 g ).

📐 Step-by-Step Calculation Guide

  1. Find the mass of oxygen reacted:
    Mass of O = Total mass of oxide - Mass of element X
    Mass of O = 0.790 g - 0.541 g = 0.249 g
  2. Calculate the moles of oxygen atoms (O):
    Moles of O = 0.249 g / 16.0 g mol⁻¹ = 0.01556 mol
  3. Use the stoichiometry of X₂O₃ to find moles of X:
    From the formula X₂O₃, 3 moles of O react with 2 moles of X.
    Moles of X = Moles of O × (2 / 3)
    Moles of X = 0.01556 mol × (2 / 3) = 0.01037 mol
  4. Calculate the molar mass (Ar) of element X:
    Ar of X = Mass of X / Moles of X
    Ar of X = 0.541 g / 0.01037 mol = 52.17 g mol⁻¹
  5. Identify element X:
    Comparing 52.17 g mol⁻¹ to the periodic table points directly to Chromium ( Cr , Ar = 52.0). Therefore, option B is correct.

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), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.