OCR A-Level Chemistry Unified chemistry (03), June 2017: Question 4

19 marks · Hard difficulty · Structured Questions

Calculate the molar mass and determine the alkyl group of weak acid A using titration data, write reaction equations for its reactions with magnesium and concentrated sulfuric acid, and calculate the mass of chromium(III) picolinate in a tablet.

Practise this question

Question

Question text

4 This question is about weak acids.

(a) Compound A is a weak monobasic acid.

A student is supplied with a 250.0 cm3 solution prepared from 2.495 g of A.

The student titrates 25.0cm3 samples of this solution with 0.0840 moldm−3 NaOH in the burette.

The student carries out a trial, followed by the three further titrations. The diagrams show the

initial burette readings and the final burette readings for the student’s three further titrations.

All burette readings are measured to the nearest 0.05 cm3.

Titration 1 Titration 2 Titration 3

Initial reading Final reading Initial reading Final reading Initial reading Final reading

0 23 23 45 9 32

1 24 24 46 10 33

2 25 25 47 11 34

(i) Record the student’s readings and the titres in an appropriate format.

Calculate the mean titre that the student should use for analysing the results.

mean titre = … cm3 [4]

(ii) The structure of compound A is shown below.

compound A

Compound A has four optical isomers.

Using this information and the student’s results, answer the following.

• Determine the molar mass of A and the formula of the alkyl group R.

• Draw the structure of compound A and label any chiral carbon atoms with an

asterisk*.

Show all your working.

[6]

(b) The structural formula of compound A is repeated below.

compound A

Two reactions of compound A are carried out.

Suggest an equation for each reaction and state the type of reaction.

In your equations, draw structures for organic compounds.

You can use R for the alkyl group.

(i) Magnesium ribbon is added to a solution of compound A.

Gas bubbles are seen and the magnesium slowly dissolves.

Equation

Type of reaction … [3]

(ii) Compound A is heated with a few drops of concentrated sulfuric acid as a catalyst.

A cyclic ‘dimer’ of compound A forms.

Equation

Type of reaction … [3]

weak acid, picolinic acid.

N N

O O O

Chromium(III) picolinate is used in tablets as a nutritional supplement for chromium.

(i) Draw the structure of the ligand in chromium(III) picolinate.

[1]

(ii) A typical tablet of chromium(III) picolinate contains 200μg of chromium.

Calculate the mass, in g, of chromium(III) picolinate in a typical tablet.

1 μg = 10−6 g.

Give your answer to three significant figures.

mass = … g [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

4 (a) (i) Burette readings 4

3 Table not required

Final (reading)/cm 23.15 45.95 32.45

Initial (reading)/cm3 0.60 23.15 10.00 ALLOW initial reading before final reading

Correct titration results recorded with initial and final

readings, clearly labeled

AND all readings recorded to two decimal places with

last figure either 0 or 5

Titres

Titre/cm3 22.55 22.80 22.45

ALLOW ECF

Correct subtractions to obtain final titres to 2 DP

Units

Units of cm3 for initial, final and titres

ALLOW units with each value

ALLOW brackets for units, i.e. (cm3)

Mean titre

22.55 + 22.45 3 ALLOW ECF from incorrect concordant titres

mean titre = = 22.50 OR 22.5 cm

i.e. using concordant (consistent) titres

(a) (ii) ALLOW 3SF or more throughout 6

IGNORE trailing zeroes, e.g. ALLOW 0.084 for 0.0840 ALLOW ECF from incorrect mean titre in 4a(i)

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

22.50 e.g. From 22.60 cm3 (mean of all 3 titres in (i),

n(NaOH) = 0.0840 = 1.89 10–3 (mol)

1000 n(NaOH) = 1.8984 10–3 (mol)

n(A) in 250 cm3 = 10 1.89 10–3 = 1.89 10–2 (mol) ALLOW ECF from incorrect n(NaOH)

2.495 –1

M(A) = –2 = 132 (g mol ) ALLOW ECF from incorrect n(A)

1.89 10

M(alkyl group) (= 132 – 75) = 57 ALLOW ECF from incorrect M(A) – 75

R = C4H9 ALLOW ECF for alkyl group closest to

calculated M(alkyl group),

ALLOW alkyl group in drawn structure with straight e.g. for M = 45, ALLOW C3H7 (43)

chain or branch(es) in wrong position,

e.g. for R = C4H9, CH3CH2CH2CH2 OR (CH3)3C

ALLOW correct structural OR skeletal OR

Structure with chiral carbon atoms identified (see * below) displayed formula OR mixture of the above as

long as non-ambiguous

IGNORE poor connectivity to OH groups

Given in question

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

Common error for 4 marks max

25.00 instead of 22.50 and scaling by 10

2.10 10–3 2.10 10–2

118.81 43.81 C3H7

25.00 instead of 22.50 and scaling by 22.50

2.10 10–3 2.33 10–2

106.93 31.93 C2H5

No structure with 2 chiral centres possible

(b) (i) 3 ALLOW correct structural OR skeletal OR

displayed formula OR mixture of the above as

long as non-ambiguous

Equation ALLOW

2HOCH(R)COOH + Mg (HOCH(R)COO)2Mg + 2HOCH(R)COOH + Mg

H 2HOCH(R)COO– + Mg2+ + H

ALLOW multiples

Organic product

IGNORE poor connectivity to OH groups

Balance Given in question

Type of reaction

Redox

(b) (ii) Equation 3 ALLOW correct structural OR skeletal OR

displayed formula OR mixture of the above as

long as non-ambiguous

ALLOW 1 mark of the 2 equation marks for

formation of ‘3 ring’ with balanced equation:

Organic product

Balance ALLOW condensation polymerisation

ALLOW addition–elimination

Type of reaction

Condensation OR esterification IGNORE elimination

IGNORE dehydration

charge outside, i.e.

ALLOW ring (Kekulé structure)

If answer = 1.61 10–3 award 2 marks

200 10–6

Note: = 3.85 10–6 (at least 3 SF)

M = 418(.0) (g mol–1) OR n(Cr) = 3.85 10–6 (mol) 52.0

Mass = 3.85 10–6 418.0 = 1.61 10–3 g ALLOW ECF from incorrect M OR n(Cr)

ALLOW 3 SF up to calculator value correctly

rounded

Total 19

How to answer it

Weak Acids, Titration Calculations & Organic Functional Group Chemistry

🔍 What this question tests

This multi-topic OCR A-Level Chemistry question tests your ability to process experimental titration data, determine molar mass and alkyl group structures using stoichiometry, identify optical isomers and chiral centres, write balanced organic equations with mechanisms/reaction types (redox and condensation/esterification), and perform complex stoichiometric calculations involving ligands and molar masses in pharmaceuticals.

Part (a)(i) — Titration Data Processing and Mean Titres

Recording Data & Calculating Concordant Titres

✅ Correct Answers

Initial Readings: 0.60 cm³, 23.15 cm³, 10.00 cm³
Final Readings: 23.15 cm³, 45.95 cm³, 32.45 cm³
Titres: 22.55 cm³, 22.80 cm³, 22.45 cm³
Mean Titre: 22.50 cm³ (using concordant titres 1 and 3: (22.55 + 22.45) / 2)

💡 Key Knowledge

All burette readings must be recorded to 2 decimal places, ending in either .00 or .50 based on precision.
Only concordant titres (usually within 0.10 cm³ of each other) should be averaged for the mean titre. Titration 2 (22.80 cm³) is discordant and discarded.

🧠 Exam Technique & Examiner Comments

Examiners heavily penalize inconsistent precision in tables. Make sure every single burette entry has two decimal places. Remember that mean titre calculations must state clearly which runs were averaged to earn credit.

Part (a)(ii) — Stoichiometry, Molar Mass, and Chiral Centres

Determining Molar Mass of Compound A and Alkyl Group R

📐 Step-by-Step Calculation

Moles of NaOH: n(NaOH) = 0.0840 × (22.50 / 1000) = 1.89 × 10⁻³ mol
Moles of Acid A in 25.0 cm³: Since it's a monobasic acid, 1:1 stoichiometry means n(A) in 25.0 cm³ = 1.89 × 10⁻³ mol
Moles of Acid A in whole flask (250 cm³): 10 × 1.89 × 10⁻³ = 1.89 × 10⁻² mol
Molar Mass M(A): M = mass / moles = 2.495 g / 1.89 × 10⁻² mol = 132 g mol⁻¹
Alkyl Group R Mass: Subtract known groups from compound A formula (HO-CH(R)-COOH = C₂HO₃R + H). M(R) = 132 - 75 = 57, giving formula C₄H₉ .

❌ Common Errors & Traps

Scaling Factor Trap: Forgetting to scale up the moles from the 25.0 cm³ aliquot to the 250 cm³ volumetric flask volume (multiplying by 10).
Incorrect Titre Averaging: Using the discordant 22.80 cm³ value instead of averaging 22.55 and 22.45 cm³.

Mark Scheme Guidance: Full marks awarded for correct final structure of compound A showing C₄H₉ (such as a butyl group branch) with chiral carbons explicitly labelled with asterisks (*).

Part (b) — Functional Group Reactions of Compound A

Reactions with Magnesium and Sulfuric Acid

✅ (b)(i) Magnesium Ribbon Added

Equation: 2HOCH(R)COOH + Mg → (HOCH(R)COO)₂Mg + H₂

Type of Reaction: Redox (or Acid + Metal)

✅ (b)(ii) Cyclic Dimer Formation

Equation: 2HOCH(R)COOH ⇌ Cyclic Dimer + 2H₂O (forming a cyclic ester ring structure)

Type of Reaction: Condensation (or Esterification)

💡 Key Knowledge

Carboxylic acids react with reactive metals (like Mg) to produce a carboxylate salt and hydrogen gas (observed via effervescence/bubbles). Heating with concentrated H₂SO₄ acts as a dehydrating catalyst, promoting esterification/condensation to form cyclic dimers with loss of water.

Part (c) — Chromium(III) Picolinate Complexes and Calculations

Ligand Structure & Tablet Mass Determination

✅ (c)(i) Structure of the Ligand

The ligand is the deprotonated picolinate ion derived from picolinic acid: a pyridine ring with a carboxylate group ( -COO⁻ ) at position 2, showing the negative charge on the oxygen atom.

📐 (c)(ii) Tablet Mass Calculation Steps

Moles of Cr: n(Cr) = (200 × 10⁻⁶ g) / 52.0 g mol⁻¹ = 3.846 × 10⁻⁶ mol
Molar Mass of Complex: Cr(C₆H₄NO₂)₃ has M = 418.0 g mol⁻¹
Mass of Complex: Mass = 3.846 × 10⁻⁶ mol × 418.0 g mol⁻¹ = 1.607 × 10⁻³ g
Final Answer (3 SF): 1.61 × 10⁻³ g

🧠 Exam Technique & Significant Figures

Always pay close attention to standard form conversions ( 1 µg = 10⁻⁶ g ). Ensure your final numerical answer is rounded cleanly to three significant figures as requested by the command words.

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 5: Physical chemistry and transition elements · Module 6: Organic chemistry and analysis · Practical Activity Groups · 5.3 Transition elements · PAG 2: Acid-base titration · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis · 4.2 Alcohols, haloalkanes and analysis · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.