OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify a Group 2 metal given the mass of the metal reacted and the mass of the chloride formed.

Practise this question

Question

Multiple choice question 8 asking to identify Group 2 metal M from the reaction of 3.528 g of M with excess chlorine to form 9.775 g of chloride. Options are A: magnesium, B: calcium, C: strontium, D: barium, with an answer box provided.
Question text

8 3.528 g of a Group 2 metal, M, is reacted with an excess of chlorine.

The reaction forms 9.775 g of a chloride.

What is metal M?

A magnesium

B calcium

C strontium

D barium

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 8 showing the correct answer is B, worth 1 mark.

8 B 1 AO2.6

How to answer it

Identifying a Group 2 Metal from Mass Data

What this question tests

This question assesses your ability to apply moles calculations to reacting masses, set up stoichiometric ratios involving Group 2 elements and halogens, and determine relative atomic mass (Ar) to identify an unknown element.

Question 8 Multiple Choice Solution

Full Worked Answer & Breakdown

✅ Correct Answer: B (calcium)

By determining the molar mass of metal M through mass conservation and mole ratios, we find its relative atomic mass is approximately 40.1 g mol⁻¹, which corresponds to calcium.

💡 Key Knowledge

  • Group 2 metals form ionic chlorides with the general formula MCl₂ .
  • Mass of chlorine reacted = Mass of chloride − Mass of metal M .
  • Relate moles of metal to moles of chlorine or chloride to find the unknown relative atomic mass.

🧠 Exam Technique

In multiple-choice questions involving unknown elements, you don't need a full written justification. However, writing out the conservation of mass and setting up a quick algebraic expression or mole ratio prevents costly arithmetic slip-ups.

📐 Step-by-Step Calculation

  1. Find the mass of chlorine that reacted:
    Mass(Cl₂) = 9.775 g (chloride) − 3.528 g (metal) = 6.247 g
  2. Calculate the moles of chlorine atoms / ions:
    Moles of Cl = 6.247 / 35.45 = 0.17622 mol
  3. Use the stoichiometry of a Group 2 chloride ( MCl₂ ):
    The ratio of metal to chloride ions is 1 : 2. Therefore, moles of metal M = Moles of Cl / 2 = 0.17622 / 2 = 0.08811 mol
  4. Calculate the molar mass (Ar) of metal M:
    Ar(M) = Mass / Moles = 3.528 g / 0.08811 mol = 40.04 g mol⁻¹
  5. Match with Periodic Table options:
    Magnesium (24.3), Calcium (40.1), Strontium (87.6), Barium (137.3). 40.04 matches calcium (B).

❌ Common Errors & Calculation Traps

  • Stoichiometry errors: Treating the formula as MCl instead of MCl₂ because Group 2 metals form 2+ ions while chlorine forms 1- ions.
  • Using molar mass of Cl₂ directly on the metal ratio: Forgetting that each mole of MCl₂ contains two moles of chlorine atoms, throwing off the 1:2 mole ratio.
Mark Scheme Allocation: 1 mark awarded for selecting B (AO2.6: Applying chemical knowledge and quantitative skills to unfamiliar contexts).

Topics

Module 3: Periodic table and energy · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.