Edexcel A-Level Chemistry AS Paper 2, June 2025: Question 5

9 marks · Medium difficulty · Short Open Response

Use mass spectrometry and infrared spectroscopy data to identify an unbranched carboxylic acid, explain limitations of IR alone, and account for peak broadening due to hydrogen bonding.

Practise this question

Question

Question 5 shows two spectra for an unbranched carboxylic acid. Part (a) shows a mass spectrum with peaks at m/z values including a base peak near 60, prominent peaks around 73 and 87, and the molecular ion peak at m/z = 116. Sub-questions ask for the relative molecular mass, the formula of a fragment ion at m/z = 15, and the name or formula of the carboxylic acid. Part (b) shows the infrared spectrum with a broad absorption between 3300 and 2500 cm⁻¹ and a sharp strong peak at around 1715 cm⁻¹. Sub-questions ask why IR alone cannot uniquely identify the carboxylic acid, to identify the bond absorbing at 1725–1700 cm⁻¹, and to explain why the peak at 3300–2500 cm⁻¹ is broad with reference to intermolecular forces.

Mark scheme

Show the mark scheme Mark scheme for Question 5: 5(a)(i) awards 1 mark for 116 (ignoring units). 5(a)(ii) awards 2 marks: 1 mark for CH3 group identified, 1 mark for positive charge on the ion ([CH3]+ scores both marks). 5(a)(iii) awards 1 mark for hexanoic acid or C5H11COOH (or C6H12O2). 5(b)(i) awards 2 marks: 1 mark for stating IR gives information about functional groups/bonds present, and 1 mark for stating it does not easily give information about the number of carbon atoms/chain length. 5(b)(ii) awards 1 mark for carbonyl group or C=O. 5(b)(iii) awards 2 marks: 1 mark for OH / O-H / hydroxyl group, and 1 mark for hydrogen bonding.

How to answer it

Mass Spectrometry & IR Spectroscopy of a Carboxylic Acid

📌 What this question tests

This question assesses core analytical skills across two major spectroscopic techniques in organic chemistry:

  • Mass Spectrometry: Locating the molecular ion peak ( M⁺ ) to determine relative molecular mass, formulating charged fragment ions, and deducing unbranched molecular structures.
  • Infrared (IR) Spectroscopy: Characteristic absorption ranges for functional groups (specifically carboxylic acid C=O and O–H bonds).
  • Spectroscopic Limitations: Explaining why IR absorption bands identify functional groups rather than entire chain lengths.
  • Intermolecular Forces: Explaining spectral feature broadening (specifically hydrogen bonding in carboxylic acids).
Part (a)(i)

Relative Molecular Mass from Mass Spectrum

Identifying the molecular ion peak (M⁺)

✅ Correct Answer

116

Mark Scheme: 116 (Ignore any units). [1 mark]

🧠 Exam Technique

  • Scan to the furthest right-hand side of the spectrum for the highest significant m/z peak.
  • Look carefully at the graduation marks between 110 and 120: each major division is 10, with tick marks representing individual units. The last significant peak sits exactly at 116.
  • Relative molecular mass ( Mᵣ ) is a dimensionless ratio—do not add units like "g" or "g mol⁻¹".
Part (a)(ii)

Formula of a Fragment Ion at m/z = 15

Writing fragment formulas with proper charges

✅ Correct Answer

[CH₃]⁺  or  CH₃⁺

Mark Scheme:
• Mark 1: CH₃ group identified [1 mark]
• Mark 2: Positive charge on the ion [1 mark]
Note: [CH₃]⁺ scores both marks directly. Brackets are optional.

❌ Common Errors

  • Omitting the positive charge: Writing just CH₃ or •CH₃ (radical). A mass spectrometer only detects charged ions! Omitting the charge loses Mark 2.
  • Writing incorrect atoms: E.g., NH⁺ (which also equals 15), but the molecule is stated to be a carboxylic acid (contains only C, H, and O).
Part (a)(iii)

Identifying the Carboxylic Acid

Deducing molecular formula and IUPAC name

✅ Correct Answer

hexanoic acid  or  CH₃(CH₂)₄COOH / C₅H₁₁COOH / C₆H₁₂O₂

Mark Scheme: Allow name or any correct formula. If both name and formula are provided, both must be correct. [1 mark]

📐 Step-by-Step Calculation

  1. General formula of saturated mono-carboxylic acid: CₙH₂ₙO₂
  2. Set up mass equation:
    (12 × n) + (1 × 2n) + (16 × 2) = 116
  3. Solve for n:
    14n + 32 = 116 ⇒ 14n = 84 ⇒ n = 6
  4. Since the question explicitly states unbranched, the 6-carbon carboxylic acid is hexanoic acid.
Part (b)(i)

Limitations of IR Spectroscopy Alone

Explaining why an IR spectrum alone cannot identify the specific carboxylic acid

✅ Correct Answer

  • The infrared spectrum only provides information about the bonds / functional groups present in the molecule (e.g. C=O and O–H ). [1 mark]
  • It does not give information about the number of carbon atoms / carbon chain length / molar mass. [1 mark]
Mark Scheme:
• Allow 'functional groups' for bonds.
• Accept chain length / molecular formula / molar mass.
• Also accept: "All carboxylic acids have the same/similar functional groups / absorption peaks" or "It is difficult to match the fingerprint region without a reference spectrum". [2 marks]

🧠 Top-Grade Examiner Tip

Two distinct ideas are required for the 2 marks:

  1. State what IR does show: identifies functional groups / characteristic bonds.
  2. State what IR does not show: does not tell you the size of the carbon chain or the molar mass.

Vague statements like "peaks don't tell us which acid it is" will not score!

Part (b)(ii)

Absorption at 1725–1700 cm⁻¹

Identifying the characteristic carbonyl peak

✅ Correct Answer

C=O  or  carbonyl group

Mark Scheme: Award name or formula of group. Do not award "carboxylic acid" alone. If both name and formula are given, name must not contradict (e.g. do not call it ketone). [1 mark]

❌ Common Errors

  • Naming the compound instead of the bond: The question asks to "Identify the bond". Answering "carboxylic acid" fails to identify the bond and scores zero.
  • Writing C–O: Be careful with bond multiplicity; the sharp intense peak at ~1715 cm⁻¹ is specifically the double bond C=O .
Part (b)(iii)

Broad Peak in the Range 3300–2500 cm⁻¹

Intermolecular hydrogen bonding in carboxylic acids

✅ Correct Answer

  • The peak is due to the O–H (hydroxyl) bond / group. [1 mark]
  • Carboxylic acid molecules form hydrogen bonds (with each other / form dimers). [1 mark]
Mark Scheme:
• M1: Identifies O–H / hydroxyl bond (do not award just "-OH" unless clearly indicating the bond between O and H).
• M2: Hydrogen bonded / intermolecular hydrogen bonding.
• Note: M1 is needed to score M2. [2 marks]

💡 Key Chemistry Concept

Why does hydrogen bonding make an IR peak broad?

  • In liquid carboxylic acids, molecules form extensive hydrogen-bonded dimers.
  • Hydrogen bonding weakens the covalent O–H bond to varying extents across different molecules.
  • This distribution of bond strengths broadens the frequencies at which the O–H bonds vibrate, creating the wide, "jagged" absorption trough spanning 3300 to 2500 cm⁻¹ (overlapping the C–H stretches).

Topics

Organic Chemistry · Physical Chemistry · Topic 7: Modern Analytical Techniques I · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.