Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 8

6 marks · Hard difficulty · Calculations

Deduce the molecular formula of ferrocene using combustion analysis data and mass spectrometry data.

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Question

Combustion train apparatus diagram showing pure oxygen passing over 0.830 g of heated ferrocene. The gaseous products pass into a first U-tube filled with solid absorbing water, then a second U-tube filled with solid absorbing carbon dioxide, with unused oxygen exiting. A table gives the mass before and after combustion: U-tube for water increases from 36.34 g to 36.74 g; U-tube for carbon dioxide increases from 35.28 g to 37.23 g. The text states ferrocene contains iron, carbon, and hydrogen only, and mass spectrometry shows a molecular ion peak at m/z = 186. The prompt asks to deduce the molecular formula of ferrocene for 6 marks.

Mark scheme

Show the mark scheme Mark scheme for Question 8 (6 marks). Marking points include: 1) Calculating mass of water (0.40 g) and carbon dioxide (1.95 g); 2) Calculating moles of water (0.0222 mol) and moles of hydrogen atoms (0.0444 mol); 3) Calculating moles of carbon dioxide/carbon (0.0443 mol); 4) Calculating the mass of iron (0.830 - (0.0444 + 0.0443*12) = 0.254 g) and moles of iron (0.00455 mol); 5) Calculating the empirical formula ratio Fe:C:H = 1:9.8:9.8 giving FeC10H10; 6) Comparing empirical formula mass (185.8) with the molecular mass of 186 to deduce the molecular formula is FeC10H10. An alternative pathway calculates moles of ferrocene first.

How to answer it

Determining the Molecular Formula of Ferrocene by Combustion Analysis

📌 What this question tests

Combustion analysis quantitative calculations: finding masses of products from absorbent U-tubes, converting mass to moles of water and carbon dioxide, determining the moles of constituent hydrogen and carbon atoms, using conservation of mass to find the remaining element (iron), finding the empirical formula, and using the molecular ion peak (m/z) from mass spectrometry to deduce the final molecular formula.

Question 8 (6 Marks)

Deduce the molecular formula of ferrocene using all of the data

📐 Step-by-Step Calculation (Method 1: Empirical Formula Route)

  1. Find mass of H₂O and CO₂ produced:
    Mass of H₂O = 36.74 − 36.34 = 0.40 g
    Mass of CO₂ = 37.23 − 35.28 = 1.95 g
    awarded for both correct masses.
  2. Find moles of H₂O and H atoms:
    Moles of H₂O = 0.40 / 18.0 = 0.02222 mol
    Each H₂O molecule contains 2 H atoms:
    Moles of H = 0.02222 × 2 = 0.04444 mol
    awarded for moles of H₂O and moles of H.
  3. Find moles of CO₂ and C atoms:
    Each CO₂ molecule contains 1 C atom:
    Moles of C = Moles of CO₂ = 1.95 / 44.0 = 0.04432 mol
    awarded for moles of CO₂ / C.
  4. Find mass and moles of Iron (Fe):
    Mass of H = 0.04444 mol × 1.0 g mol⁻¹ = 0.04444 g
    Mass of C = 0.04432 mol × 12.0 g mol⁻¹ = 0.53184 g
    Total mass of (C + H) = 0.04444 + 0.53184 = 0.57628 g
    Mass of Fe = Total mass − mass of (C + H) = 0.830 − 0.57628 = 0.25372 g
    Moles of Fe = 0.25372 / 55.8 = 0.004547 mol
    awarded for mass of (C + H) and moles of Fe.
  5. Determine Empirical Formula:
    Divide each molar value by the smallest number of moles (0.004547):
    Fe : C : H = (0.004547 / 0.004547) : (0.04432 / 0.004547) : (0.04444 / 0.004547)
    Ratio = 1 : 9.75 : 9.77 ≈ 1 : 10 : 10
    Empirical Formula = FeC₁₀H₁₀ (or C₁₀H₁₀Fe )
    awarded for obtaining the correct empirical formula.
  6. Deduce Molecular Formula using m/z = 186:
    Empirical formula mass = 55.8 + (10 × 12.0) + (10 × 1.0) = 185.8 ≈ 186
    Since the empirical formula mass equals the molecular ion peak (m/z = 186), the molecular formula is identical to the empirical formula: FeC₁₀H₁₀.
    awarded for comparing empirical mass to 186 and stating final molecular formula.

📐 Alternative Method (Direct Moles of Ferrocene Route)

  • Moles of Ferrocene sample: 0.830 g / 186 g mol⁻¹ = 0.004462 mol (Mark 1)
  • Compare mole ratios directly: Ferrocene : C : H = 0.004462 : 0.04432 : 0.04444 ≈ 1 : 10 : 10 (Mark 2)
  • Find mass of Fe: Mr of C₁₀H₁₀ = (10 × 12) + 10 = 130; Remaining mass = 186 − 130 = 56 (Mark 3)
  • Deduce Fe count: 56 / 55.8 ≈ 1 Fe atom; therefore Molecular Formula = FeC₁₀H₁₀ (Mark 4)

✅ Final Answer

Molecular formula:

FeC₁₀H₁₀   (or C₁₀H₁₀Fe)

Accept symbols in any order as long as subscript ratios are 1 : 10 : 10.

💡 Key Knowledge

  • Combustion products: All hydrogen in the sample becomes H₂O; all carbon becomes CO₂.
  • Hydrogen multiplier: 1 mol H₂O contains 2 moles of H atoms.
  • Third element by difference: The mass of Fe cannot be measured directly by combustion; it must be found by subtracting the masses of C and H from the total initial sample mass.
  • Mass spectrum: The molecular ion peak ( m/z = 186 ) gives the relative molecular mass (Mr) of the whole compound.

🧠 Exam Technique & Strategy

  • Keep full precision: Do not round intermediate mole values too early (e.g. keep 0.04444 instead of rounding to 0.04). Premature rounding skews the final 1:10:10 ratio significantly.
  • Check atom masses: Use precise atomic masses from the periodic table: Fe = 55.8, C = 12.0, H = 1.0.
  • Answer the specific prompt: The question asks for the molecular formula, not just the empirical formula. You must explicitly relate your empirical mass (185.8) to the given m/z (186).

❌ Common Errors & Pitfalls

  • Forgetting the '× 2' for Hydrogen: Calculating moles of H as equal to moles of H₂O (0.0222 mol instead of 0.0444 mol). This leads to an incorrect ratio of FeC₁₀H₅.
  • Subtracting moles instead of masses: Trying to find Fe by subtracting moles of C and H from the 0.830 g starting mass. You can only subtract mass from mass!
  • Using Ar(O) mistakenly: Oxygen absorbed in the tubes comes from the oxygen gas stream, NOT the ferrocene sample. Ferrocene contains only Fe, C, and H.

Topics

Physical Chemistry · Organic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 7: Modern Analytical Techniques I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.