AQA A-Level Chemistry Paper 2, 2019: Question 10

10 marks · Medium difficulty · State/Explain/Numerical

Deduce the molecular formula of a dicarboxylic acid with Mr = 118, calculate the mass needed in a titration, analyze the 1H NMR spectrum of isomeric diols, and explain how high resolution mass spectrometry distinguishes them.

Practise this question

Question

Question 10 is split into five parts: 10.1 asks to deduce the molecular formula of a dicarboxylic acid with Mr = 118 to the nearest whole number (3 marks). 10.2 is a titration calculation where a solution made up to 250 cm3 requires 21.60 cm3 of 0.109 mol dm-3 NaOH to neutralise 25.0 cm3 samples; students must find the initial mass of acid to the appropriate number of significant figures (4 marks). 10.3 shows the structural formula of hexane-2,5-diol and asks for the number of peaks in its 1H NMR spectrum (1 mark). 10.4 asks to draw the structure of an isomeric diol C6H14O2 whose 1H NMR spectrum consists of two singlet peaks (1 mark). 10.5 asks to state why the dicarboxylic acid and the diol isomers can be distinguished by high resolution mass spectrometry (1 mark).
Question text

10 Some compounds with different molecular formulas have the same relative molecular

mass to the nearest whole number.

10.1 A dicarboxylic acid has a relative molecular mass of 118, to the nearest whole

number.

Deduce the molecular formula of the acid.

[3 marks]

Molecular formula

10.2 A student dissolved some of the dicarboxylic acid from Question 10.1 in water and

made up the solution to 250 cm3 in a volumetric flask.

In a titration, a 25.0 cm3 sample of the acid solution needed 21.60 cm3 of

0.109 mol dm−3 sodium hydroxide solution for neutralisation.

Calculate the mass, in g, of the dicarboxylic acid used.

Give your answer to the appropriate number of significant figures.

[4 marks]

25 Mass g

10.3 Compounds with molecular formula C6H14O2 also have a relative molecular mass of

118 to the nearest whole number. These include the diol shown.

Deduce the number of peaks in the 1H NMR spectrum of this diol.

[1 mark]

10.4 Draw the structure of a different diol also with molecular formula C6H14O2 that has a

1H NMR spectrum that consists of two singlet peaks.

[1 mark]

10.5 The dicarboxylic acid in question 10.1 and the isomers of C6H14O2 in Questions

10.3 and 10.4 all have a relative molecular mass of 118

State why the dicarboxylic acid can be distinguished from the two diols by

high resolution mass spectrometry using electrospray ionisation.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for Question 10: 10.1 gives M1 for (COOH)2 = C2H2O4 = 90, M2 for remainder 118 - 90 = 28 or C2H4, M3 for molecular formula C4H6O4. 10.2 gives M1 for moles of NaOH = 2.3544 x 10^-3 mol, M2 for moles of H2A in 25 cm3 = 1.177 x 10^-3 mol, M3 for moles in 250 cm3 = 1.177 x 10^-2 mol, and M4 for mass = 1.39 g (to 3 sig figs). 10.3 gives 1 mark for 4 peaks. 10.4 gives 1 mark for 2,3-dimethylbutane-2,3-diol drawn either displayed or skeletal. 10.5 gives 1 mark for stating that their precise relative molecular masses are different.

Question Answers Additional Comments/Guidelines Mark

(COOH)2 = C2H2O4= 90 M1

118 - 90 = 28 OR C2H4 M2

10.1

C4H6O4 Must be molecular formula M3

Structural formula can score M1 & M2

Amount NaOH = (21.60 × 10 3 ) × 0.109

= 2.3544 × 10 3 mol M1 for answer (to 3sfs min) M1

Amount H A in 25cm3 = 1.177 × 10 3 mol M2 = 0.5 × M1 M2

10.2 2

Amount H A in 250 cm3 = 1.177 × 10 2 mol M3 = M2 × 10 M3

Mass = 1.39 g (Must be 3sf) M4 = answer to (M3 × 118) and must be 3sf M4

G 10.3 4 or four 1

CH3 CH3

10.4 CH3 C C CH3 1

OH OH OR OH OH

The precise (relative molecular) masses are different or wtte Allow Mr are different to 2 or more or several dp

Ignore different molecular formula

10.5 1

Ignore accuracy

Penalise fragments

How to answer it

Dicarboxylic Acids, Titration Calculations & Diol Isomers

What this question tests

  • Formula deduction: Finding a molecular formula from a known relative molecular mass ( Mr ) and functional group requirements.
  • Quantitative titration calculations: Multi-step volumetric calculations involving reacting ratios (1:2 acid-base), aliquot scaling, and final mass to appropriate significant figures.
  • 1H NMR interpretation: Identifying molecular symmetry to predict the number of chemical environments (peaks).
  • Structural isomerism: Constructing a symmetric diol isomer showing only single sharp peaks (singlets with no adjacent protons).
  • High-Resolution Mass Spectrometry (HRMS): Understanding how precise isotopic masses differentiate compounds having identical nominal integer masses.
Question 10.1

Deducing the Molecular Formula of the Dicarboxylic Acid

3 Marks

✅ Correct Answer & Deduction

Molecular formula: C4H6O4

Mark Scheme Breakdown:
• M1: Identifying two carboxylic acid groups: 2 × COOH = C2H2O4 = 90
• M2: Remaining mass: 118 − 90 = 28 (which corresponds to C2H4)
• M3: Combining to molecular formula: C4H6O4

❌ Common Errors

  • Writing a structural formula like HOOC-CH2-CH2-COOH on the final answer line instead of the requested molecular formula. (Can score M1 & M2, but forfeits M3!)
  • Miscalculating the mass of two -COOH groups (forgetting that a dicarboxylic acid contains 4 oxygen atoms and 2 acidic hydrogen atoms).
Question 10.2

Titration Calculation: Finding Mass of Dicarboxylic Acid

4 Marks

📐 Step-by-Step Calculation

Step 1: Calculate moles of NaOH used in the titration
Moles = concentration × volume (in dm3)
Moles of NaOH = 0.109 mol dm−3 × (21.60 / 1000) dm3 = 2.3544 × 10−3 mol
Step 2: Determine moles of acid in the 25.0 cm3 aliquot (Stoichiometry)
The acid is dicarboxylic (H2A), requiring 2 moles of NaOH per mole of acid:
H2A + 2NaOH → Na2A + 2H2O
Moles of H2A in 25.0 cm3 = (2.3544 × 10−3) ÷ 2 = 1.1772 × 10−3 mol
Step 3: Scale up to the full 250 cm3 volumetric flask
Scaling factor = 250 cm3 / 25.0 cm3 = 10
Moles of H2A in 250 cm3 = 1.1772 × 10−3 × 10 = 1.1772 × 10−2 mol
Step 4: Calculate original mass of dicarboxylic acid used
Mass = moles × Mr = (1.1772 × 10−2 mol) × 118 g mol−1 = 1.3891 g
To appropriate significant figures (3 s.f.): 1.39 g

🧠 Exam Technique: Significant Figures

The prompt asks for "the appropriate number of significant figures":

  • Concentration of NaOH is given to 3 sig figs ( 0.109 ).
  • Volumes are given to 3 or 4 sig figs ( 250 , 25.0 , 21.60 ).
  • Therefore, the limiting precision is 3 sig figs. The final answer must be stated as 1.39 g.

❌ Common Calculation Traps

  • Missing the 1:2 ratio: Forgetting the acid is dicarboxylic. Failing to divide NaOH moles by 2 loses M2, M3, and M4.
  • Skipping the aliquot step: Omitting the × 10 scaling factor between the 25 cm3 titre and 250 cm3 volumetric flask.
  • Rounding too early: Rounding intermediate numbers to 2 sig figs can distort the final answer away from 1.39 g.
Question 10.3

Number of Peaks in 1H NMR of Hexane-2,5-diol

1 Mark

✅ Correct Answer

4 (or four) peaks

💡 Symmetry Analysis

The molecule is symmetrical down the central C3—C4 bond:

  • Environment 1: The two terminal -CH3 groups (6H total, split into a doublet).
  • Environment 2: The two -CH- protons on carbons 2 and 5 (2H total).
  • Environment 3: The two hydroxyl -OH protons (2H total).
  • Environment 4: The central -CH2-CH2- protons on carbons 3 and 4 (4H total).
Question 10.4

Drawing an Isomer with Exactly Two Singlets in 1H NMR

1 Mark

✅ Structure: 2,3-Dimethylbutane-2,3-diol (Pinacol)

Structural Formula: (CH3)2C(OH)—C(OH)(CH3)2

How to draw this structure clearly:
• A central ethane-1,2-diol backbone: two adjacent carbons, each bonded to an -OH group.
• Each of these two central carbons is fully substituted with two -CH3 groups.
• Displayed or skeletal formula is accepted.

🧠 Deducing the Structure

  • Why only 2 peaks? The molecule must have very high symmetry, leaving only 2 proton environments.
  • Why singlets? Protons cannot have any non-equivalent protons on adjacent carbons ( n + 1 = 1 means n = 0 ).
  • All 12 methyl hydrogens are equivalent and attached to quaternary carbons (no adjacent H → singlet). Both OH protons are equivalent (singlet).
Question 10.5

Distinction by High-Resolution Mass Spectrometry (HRMS)

1 Mark

✅ Correct Answer

The precise (relative molecular) masses are different (or different to 2 or more decimal places).

💡 Why HRMS Works Here

Both compounds have nominal mass = 118:

  • Acid: C4H6O4
  • Diols: C6H14O2

Because accurate atomic masses are not exact whole numbers (e.g., 1H = 1.0078, 12C = 12.0000, 16O = 15.9949), their exact molecular masses differ at multiple decimal places.

❌ Examiner Trap / Mark Scheme Guidance

  • Do NOT just say: "They have different molecular formulas" — the question already states they have different formulas; you must state that their exact/precise masses differ.
  • Do NOT mention fragmentation: Electrospray ionisation is a "soft" ionisation method where little to no fragmentation occurs. Mentioning fragment ions is penalised.

Topics

Physical Chemistry · Organic Chemistry · 3.1.2 Amount of Substance · 3.3.6 Organic Analysis · 3.3.9 Carboxylic Acids and Derivatives · 3.3.15 Nuclear Magnetic Resonance Spectroscopy

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.