OCR A-Level Chemistry AS Depth in chemistry (02), June 2024: Question 6

6 marks · Hard difficulty · Extended Response

Determine the structure of an organic compound X containing two functional groups using percentage composition, mass spectrometry, and infrared spectroscopy data.

Practise this question

Question

An exam question providing percentage composition by mass for organic compound X (C: 40.91%, H: 4.54%, O: 54.55%), stating it does not decolourise bromine water, and showing a mass spectrum with peaks including a molecular ion peak at m/z = 88 and an infrared spectrum showing a broad O-H stretch around 3000 cm-1 and a C=O stretch around 1700 cm-1. Students are asked to use all the information to determine a possible structure of compound X with 6 marks.
Question text

Compound X is an organic compound with two functional groups.

Compound X has the percentage composition by mass:

C, 40.91%; H, 4.54%; O, 54.55%.

Compound X does not decolourise bromine water.

A scientist analyses compound X using mass spectrometry and infrared spectroscopy.

Mass spectrum of X

relative

intensity

10 20 30 40 50 60 70 80 90 100 110

m/z

IR spectrum of X

transmittance 50

(%)

4000 3000 2000 1500 1000 500

wavenumber / cm–1

Use all the information to determine a possible structure of compound X.

In your answer, make it clear how your conclusions are linked to the evidence. [6]

Extra answer space if required

Mark scheme

Show the mark scheme The mark scheme outlines a levels-of-response mark scheme (Levels 1 to 3, up to 6 marks) detailing indicative scientific points: calculating empirical and molecular formula (C3H4O3, Mr = 88), interpreting the IR spectrum for O-H and C=O groups, deducing the presence of a carboxylic acid and lack of C=C bonds, identifying mass spectrum fragments, and providing an accepted structure of compound X.

Question Answer Marks Guidance

6 Please refer to the marking instructions on page 5 of the 6 LOOK ON THE SPECTRA for labelled peaks.

mark scheme for guidance on how to mark this question. Indicative scientific points may include:

Level 3 (5–6 marks) 1. Empirical formula

A comprehensive description including most of the

evidence to justify the correct structure of X. Element %mass Ar moles ratio

C 40.91 12 3.41 3

There is a well-developed line of reasoning which is H 4.54 1 4.54 4

clear and logically structured. The information presented O 54.55 16 3.41 3

is relevant and substantiated Empirical formula = C3H4O3

ALLOW Alternative method using Mr of 88 i.e.

Level 2 (3–4 marks) C = 88 x (40.91/100) x 12 = 3 etc.

Explains two scientific points with few omissions OR

some aspects from all three

AND 2. Spectra and Molecular formula

an attempt at a feasible structure with either a C=O OR

COOH Mass spectrum

• molecular ion peak m/z or Mr = 88

There is a line of reasoning presented with some

• molecular formula = C3H4O3

structure. The information presented is relevant and

supported by some evidence.

IR

• peak at 2500 to 3500 cm–1 is O–H

Level 1 (1–2 marks)

• peak at 1630 to1820 cm–1 is C=O

Determines the correct empirical/molecular formula

OR

Some aspects from two scientific points are given

3. Functional groups and structure of X

There is an attempt at a logical structure with a line of

reasoning. The information is in the most part relevant • X contains a carboxylic acid

• X doesn’t decolourise Br2 so no C=C bond

0 marks – No response worthy of credit. • Mass spectrum fragment peak(s) identified e.g.

- m/z = 43 for CH CO+

- m/z = 29 CHO+

- m/z =15 due to CH +

24 • Structure of X

OR

Aspects of the communication statement might typically have

been met when evidence is presented in a logical and clear

order making good use of all the evidence given.

Some points which may be seen where communication is

good include:

• Easy to follow layout on empirical formula calculation

• Empirical formula is same as molecular formula i.e. not

given as CH1.33O

• IR peaks linked clearly to bond it refers to not just

functional groups

• Positive charge given on MS fragments

• MS fragments plausible for the molecular formula

determined.

• No additional irrelevant/incorrect information given

How to answer it

Organic Structure Determination from Analytical Data

OCR AS Level Chemistry • 6-Mark Extended Response

What this question tests

This synoptic 6-mark level of response question assesses your ability to combine multiple analytical techniques to deduce an organic structure. You must integrate percentage composition data (to find empirical/molecular formulas), infrared (IR) spectroscopy (to identify functional groups), mass spectrometry (to determine relative molecular mass and key fragments), and chemical test observations (to rule out specific bonding like C=C double bonds).

Question 6: Determining Structure of Compound X

Full Mark Breakdown & Guidance

✅ Correct Answer

Compound X is CH₃COCH₂COOH (oxopropanoic acid / acetoacetic acid / 3-oxobutanoic acid) or equivalent structural isomer like HCOCH₂CH₂COOH .

Molecular Formula: C₃H₄O₃

Level 3 (5–6 marks): Comprehensive description linking all pieces of evidence to justify the structure with a clear, logical line of reasoning.

📐 Step-by-Step Calculation (Empirical Formula)

  1. Assume 100g of sample:
    m(C) = 40.91 g, m(H) = 4.54 g, m(O) = 54.55 g
  2. Calculate moles:
    n(C) = 40.91 / 12.0 = 3.41 mol
    n(H) = 4.54 / 1.0 = 4.54 mol
    n(O) = 54.55 / 16.0 = 3.41 mol
  3. Find simplest whole-number ratio:
    Divide by smallest (3.41):
    C : H : O = 1 : 1.33 : 1
  4. Scale up to integers:
    Multiply by 3 to get whole numbers:
    C : H : O = 3 : 4 : 3 → Empirical formula: C₃H₄O₃

💡 Key Knowledge & Analysis

  • Mass Spec ( m/z ): Molecular ion peak ( M⁺ ) is at m/z = 88 . This matches the Mᵣ of empirical formula C₃H₄O₃ (3×12 + 4×1 + 3×16 = 88), meaning the molecular formula is also C₃H₄O₃ .
  • IR Spectroscopy: Broad absorption peak between 2500–3500 cm⁻¹ indicates an O-H bond in a carboxylic acid. Sharp peak around 1630–1820 cm⁻¹ indicates a C=O carbonyl group.
  • Chemical Test: Does not decolourise bromine water, proving the absence of an alkene ( C=C ) double bond.

🧠 Exam Technique & Communication

  • Explicitly link every conclusion to a piece of data (e.g., "The peak at 88 in the mass spectrum confirms the molecular mass is 88, proving empirical and molecular formulas are identical").
  • Include positive charges on mass spec fragments if quoted (e.g., CH₃CO⁺ at m/z = 43 or CHO⁺ at m/z = 29 ).
  • Structure your answer methodically: 1. Formula → 2. Spectra Analysis → 3. Functional Groups → 4. Final Structure.

❌ Common Errors & Pitfalls

  • Empirical to Molecular confusion: Leaving the empirical formula as CH₁.₃₃O instead of scaling it up to integers ( C₃H₄O₃ ).
  • Vague IR assignments: Stating "there is an oxygen peak" instead of specifying "O-H stretch in a carboxylic acid" or "C=O carbonyl stretch".
  • Ignoring chemical tests: Forgetting to use the bromine water information to rule out unsaturated structures.

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 2.1 Atoms and reactions · 4.2 Alcohols, haloalkanes and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.