OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 21

7 marks · Medium difficulty · Calculations

Calculate the relative atomic mass of iron from a mass spectrum, name and balance equations for iron compounds, and calculate the total volume of gas produced from the thermal decomposition of iron(III) nitrate.

Practise this question

Question

A three-part chemistry exam question about iron. Part (a) provides a table of relative abundances for isotopes 54Fe, 56Fe, 57Fe, and 58Fe, and asks to calculate the relative atomic mass to 2 decimal places. Part (b)(i) asks for the systematic name of Fe2O3, and part (b)(ii) asks to balance the equation for the reduction of Fe2O3 with carbon monoxide. Part (c) provides the thermal decomposition equation of Fe(NO3)3, its molar mass, a mass of 4.836 g, and asks to calculate the total volume of gas produced in dm3 at RTP.
Question text

21 This question is about iron.

(a) A sample of iron is isolated from a meteorite and analysed by mass spectrometry.

The mass spectrum shows peaks with the relative abundances below.

Isotope 54Fe 56Fe 57Fe 58Fe

Relative abundance 78.54% 8.88% 5.10% 7.48%

Calculate the relative atomic mass of the iron in the sample.

Give your answer to 2 decimal places.

relative atomic mass = … [2]

(b) Iron can be extracted from iron ores containing the oxide Fe2O3.

(i) What is the systematic name for Fe2O3?

… [1]

(ii) Balance the equation for the reduction of Fe2O3 with carbon monoxide.

… Fe2O3 + … CO … Fe + … CO2 [1]

(c) The iron compound, Fe(NO3)3, decomposes when heated.

The equation for the decomposition of Fe(NO3)3 is shown below.

4Fe(NO3)3(s) 2Fe2O3(s) + 12NO2(g) + 3O2(g)

Molar mass of Fe(NO ) = 241.8 g mol–1

4.836 g of Fe(NO3)3 is heated until it has completely decomposed.

Calculate the total volume of gas, in dm3, produced at RTP.

total volume of gas = … dm3 [3]

Mark scheme

Show the mark scheme The mark scheme displays the worked solutions and acceptable answers for all parts of question 21. Part (a) shows the calculation leading to 54.63 with 2 marks. Part (b)(i) gives iron(III) oxide for 1 mark. Part (b)(ii) gives the balanced equation coefficients: 1, 3, 2, 3 for 1 mark. Part (c) breaks down the 3-mark calculation for moles of Fe(NO3)3, total moles of gases NO2 and O2, and the final volume 1.8(0) dm3 using molar gas volume or ideal gas equation.

Question Answer Marks Guidance

21 (a) FIRST CHECK ANSWER ON THE ANSWER LINE 2

IF answer = 54.63 (to 2 DP) award 2 marks

(54 78.54) + (56 8.88) + (57 5.10) + (58 7.48) For 1 mark: ALLOW ECF → to 2 DP if:

100 • %s used with wrong isotopes ONCE

OR 54.6298 OR 54.630 ✓ OR

• transposed decimal places for ONE %

= 54.63 (to 2 DP) ✓

(b) (i) iron(III) oxide ✓ 1 IGNORE iron(3) oxide, iron(III) dioxide, etc

i.e. MUST be systematic

ALLOW no brackets

(b) (ii) Fe2O3 + 3 CO → 2 Fe + 3 CO2 ✓ 1 ALLOW multiples

e.g. 2 Fe2O3 + 6 CO → 4 Fe + 6 CO2

ALLOW 1 Fe2O3 ….. but NOT 0 Fe2O3 …..

(c) FIRST CHECK ANSWER ON THE ANSWER LINE 3 ALLOW ECF throughout

IF answer = 1.8(0) (dm3) award 3 marks

4.836

n(Fe(NO3)3) = 241.8 = 0.02(00) (mol) ✓ ALLOW no trailing zeroes (e.g. 0.02 for 0.0200)

n(NO2 + O2) = 0.06 + 0.015

OR 15/4 0.0200

OR 0.0750 (mol) ✓

Only award ECF using moles for NO2, O2, NO2 + O2

3 e.g. NO : 0.06 24 = 1.44 (dm3)

Total volume = 0.0750 24 = 1.8(0) (dm ) ✓ 2

O : 0.015 24 = 0.36 (dm3)

DO NOT ALLOW 0.02 24 = 0.48 dm3

0.48 dm3 is 1 mark only for whole question ALLOW

Omission of ÷4 for 1 NO2 AND/OR O2

e.g. 1.8 24 = 7.2 (dm3)

---------------------------------------------------------------------------

ALLOW use of ideal gas equation using sensible p and T

for final mark. e.g.

from 100 kPa and 293 K

nRT 0.075 8.314 293 3

→ V = p = 1000 = 1.83 dm

ALLOW 1 DP: 1.8 dm3

from 100 kPa and 298 K

nRT 0.075 8.314 298 3

→ V = p = 1000 = 1.86 dm

ALLOW 1 DP: 1.9 dm3

from 100 kPa and 273 K

nRT 0.075 8.314 273 3

→ V = p = 1000 = 1.7(0) dm

Examples of ‘sensible’ p and T:

p = 100 kPa, 101 kPa, 101,325 Pa

T = 273 – 298 K

How to answer it

Iron Chemistry & Mass Spectrometry

What this question tests

This question assesses core foundation stoichiometry and atomic structure skills: calculating relative atomic mass (Ar) from percentage abundance mass spectra, applying IUPAC nomenclature for transition metal compounds with Roman numerals, balancing reduction equations, and performing multi-step moles-to-gas-volume calculations using molar gas volume at RTP (24.0 dm³ mol⁻¹).

Question 21 (a) — [2 marks]

Relative Atomic Mass Calculation

✅ Correct Answer

54.63

Method: ((54 × 78.54) + (56 × 8.88) + (57 × 5.10) + (58 × 7.48)) / 100 = 54.6298...

💡 Key Knowledge

  • Relative atomic mass (Ar) is the weighted mean mass of an atom compared to 1/12th the mass of an atom of carbon-12.
  • For percentage abundance, always divide the sum of (isotopic mass × percentage) by 100.

🧠 Exam Technique

  • Always follow rounding instructions carefully: the question explicitly requests 2 decimal places.
  • Write out your unrounded answer first ( 54.6298 ) before final rounding to avoid intermediate rounding errors.

❌ Common Errors

  • Dividing by numbers other than 100 (e.g., dividing by 4 because there are four isotopes).
  • Failing to round to the specified 2 decimal places, losing the final mark.
Mark breakdown: 1 mark for correct working/expression; 1 mark for correct final answer (54.63).
Question 21 (b)(i) — [1 mark]

Systematic Nomenclature

✅ Correct Answer

iron(III) oxide

💡 Key Knowledge

  • Systematic names for transition metal compounds must include the oxidation state in Roman numerals inside parentheses immediately following the metal name.
  • Oxygen has an oxidation state of -II, meaning two iron atoms balance three oxygen atoms (-6 total), giving each iron an oxidation state of +III.

❌ Common Errors

  • Writing "iron oxide" without the Roman numeral (loses the mark as it is not fully systematic).
  • Using incorrect Roman numerals like iron(II) oxide or iron(IV) oxide.
Mark breakdown: 1 mark for "iron(III) oxide" (Roman numerals must be present; ignore absence of brackets in some contexts, but strict systematic IUPAC requires them).
Question 21 (b)(ii) — [1 mark]

Balancing Redox Equations

✅ Correct Answer

Fe₂O₃ + 3CO → 2Fe + 3CO₂

🧠 Exam Technique

  • Balance complex elements like iron (Fe) or oxygen in the oxide first, then balance carbon and oxygen using carbon monoxide (CO) and carbon dioxide (CO₂).

❌ Common Errors

  • Altering chemical formula subscripts (e.g., writing CO₃ instead of CO ) to force balancing.
Mark breakdown: 1 mark for the completely balanced equation (multiples are accepted, but simplest whole number ratio is standard).
Question 21 (c) — [3 marks]

Gas Volume Calculation from Thermal Decomposition

📐 Step-by-Step Calculation

  1. Find moles of solid reactant:
    Moles of Fe(NO₃)₃ = Mass / Molar Mass = 4.836 / 241.8 = 0.0200 mol
  2. Use molar ratios to find total moles of gas produced:
    From the equation, 4 moles of Fe(NO₃)₃ produce 12 moles of NO₂ and 3 moles of O₂, giving a total of 15 moles of gas (12 + 3 = 15).
    Ratio factor = 15 / 4 = 3.75.
    Total moles of gas = 0.0200 × (15 / 4) = 0.0750 mol (Alternatively: n(NO₂) = 0.06 mol, n(O₂) = 0.015 mol, total = 0.075 mol).
  3. Calculate total volume of gas at RTP:
    Volume = Moles × 24.0 dm³ mol⁻¹
    Volume = 0.0750 × 24.0 = 1.8 (dm³)

🧠 Exam Technique & Guidance

  • Read stoichiometry carefully: students frequently forgot to sum both gases produced (NO₂ and O₂) in the balanced equation.
  • Check significant figures: working cleanly leads to 1.8 or 1.80 dm³.

❌ Common Calculation Traps

  • Calculating moles of only one gas product instead of adding NO₂ and O₂ together.
  • Using 22.4 dm³ instead of the standard A-Level RTP molar gas volume of 24.0 dm³ mol⁻¹.
Mark breakdown: 1 mark for moles of Fe(NO₃)₃; 1 mark for total moles of gas using correct stoichiometry ratio; 1 mark for final calculated volume with units. ECF applies throughout.

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 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.