OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 7

1 mark · Medium difficulty · Multiple Choice

Determine the identity of element X given the mass reacted with chlorine to form a chloride with the formula XCl2.

Practise this question

Question

Multiple choice question 7 asks to identify element X when 0.24 g of X reacts with 0.0100 mol of Cl2 to form XCl2. The options are A carbon, B magnesium, C molybdenum, and D titanium. An answer box is provided at the bottom with a 1 mark allocation.
Question text

70.24 g of an element, X, is reacted with 0.0100 mol Cl2 to form a chloride with the formula XCl2.

What is element X?

A carbon

B magnesium

C molybdenum

D titanium

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme for question 7 indicates the correct answer is B, worth 1 mark.

7 B 1 AO2.6

How to answer it

Determining Identity of Element X from Stoichiometry

What this question tests

This question assesses your ability to apply the molar gas/mass relationships, interpret chemical formulae and reacting ratios (stoichiometry), and calculate relative atomic mass (Ar) to identify an unknown element from the Periodic Table.

Question 7 • Multiple Choice [1 Mark]

Full Worked Solution & Breakdown

✅ Correct Answer

B (magnesium)

Mark Awarded: 1 mark for selecting B.

💡 Key Knowledge

  • Formula given: XCl₂ means 1 mole of element X reacts with 1 mole of Cl₂ .
  • Reacting ratio is therefore 1 : 1.
  • Molar mass formula: Ar = Mass (g) / Moles (mol) .

📐 Step-by-Step Calculation

  1. Find moles of X: From the formula XCl₂ and the 1:1 reacting ratio, moles of X = moles of Cl₂ = 0.0100 mol .
  2. Calculate Ar of X:
    Ar = Mass / Moles
    Ar = 0.24 g / 0.0100 mol = 24.0 g mol⁻¹ .
  3. Identify the element: Look up relative atomic masses on the Periodic Table. Magnesium ( Mg ) has an Ar of 24.3 (approx 24).

🧠 Exam Technique

In multiple-choice calculation questions, do not waste time calculating exact decimal places if the options are distinctly spaced whole elements. Quick mental estimation or rough division immediately points toward magnesium (Ar ≈ 24) rather than carbon (12), molybdenum (~96), or titanium (~48).

❌ Common Errors & Traps

  • Incorrect Ratio Trap: Some students incorrectly assume that because there are two chlorines ( Cl₂ and XCl₂ ), they need to multiply or divide the moles of chlorine by 2. Always write out or visualize the balanced equation: X + Cl₂ → XCl₂ to confirm the 1:1 molar ratio.
  • Inverted Formula: Using Moles / Mass instead of Mass / Moles , leading to completely nonsensical Ar values.

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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.