OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 8

1 mark · Medium difficulty · Multiple Choice

Calculate the total volume of gas produced at RTP when 0.00250 mol of magnesium nitrate is thermally decomposed.

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Question

Multiple choice question 8 showing the thermal decomposition equation for magnesium nitrate: Mg(NO3)2(s) -> MgO(s) + 2NO2(g) + 0.5O2(g). Given 0.00250 mol of magnesium nitrate is decomposed, students must find the total volume of gas produced at RTP from options A (30 cm3), B (60 cm3), C (120 cm3), and D (150 cm3).
Question text

8 Magnesium nitrate, Mg(NO3)2, decomposes when heated:

Mg(NO3)2(s) MgO(s) + 2NO2(g) + ½O2(g)

0.00250 mol of Mg(NO3)2 is decomposed.

What is the volume of gas produced, measured at RTP?

A 30 cm3

B 60 cm3

C 120 cm3

D 150 cm3

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows the correct answer is D, worth 1 mark.

8 D 1 AO2.6

How to answer it

Thermal Decomposition of Magnesium Nitrate

What this question tests

This question assesses your understanding of Group 2 thermal decomposition reactions, stoichiometric mole ratios in balanced chemical equations, and the conversion between moles of gas and volume at room temperature and pressure (RTP).

Question 8 (Multiple Choice)

Full Worked Solution & Examiner Breakdown

✅ Correct Answer

D (150 cm³)

Mark Awarded: 1 / 1 (AO2.6)

💡 Key Knowledge

  • Group 2 nitrates decompose on heating to form a metal oxide, nitrogen dioxide gas, and oxygen gas.
  • One mole of any gas occupies 24.0 dm³ (or 24,000 cm³ ) at RTP.
  • State symbols matter: check which products are gases ( g ) versus solids ( s ).

🧠 Exam Technique

Do not rush multiple-choice gas volume questions. Always sum the total moles of all gaseous products from the stoichiometric coefficients before multiplying by the molar gas volume.

❌ Common Errors

  • Only counting NO₂: Forgetting to include the O₂ gas produced, leading to an incorrect total mole ratio.
  • Unit conversion traps: Forgetting to multiply by 24,000 to convert dm³ into cm³.
  • Ratio inversion: Misreading the 1 : (2 + 0.5) stoichiometric ratio.

📐 Step-by-Step Calculation

  1. Identify total moles of gas per mole of Mg(NO₃)₂:
    From the equation Mg(NO₃)₂(s) → MgO(s) + 2NO₂(g) + 0.5O₂(g) , 1 mole of Mg(NO₃)₂ produces 2 + 0.5 = 2.5 moles of gas in total.
  2. Calculate total moles of gas produced from 0.00250 mol of reactant:
    Total moles of gas = 0.00250 mol × 2.5 = 0.00625 mol
  3. Convert moles of gas to volume at RTP (in cm³):
    Volume = Moles × Molar Gas Volume ( 24,000 cm³ mol⁻¹ )
    Volume = 0.00625 mol × 24,000 cm³ mol⁻¹ = 150 cm³

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.