Edexcel A-Level Chemistry Paper 1, June 2024: Question 10

14 marks · Hard difficulty · Practical Techniques and Data Analysis

Describe the preparation of a standard solution of vitamin C, complete a redox equation, and perform back-titration calculations with sodium thiosulfate to determine vitamin C content.

Practise this question

Question

Question 10 describes an analytical procedure to determine the vitamin C content of a 2.50 g tablet. Part (a) asks students to describe how to prepare a 250.0 cm³ aqueous solution from a crushed tablet. Part (b)(i) asks to balance an ionic equation where IO3⁻ reacts with I⁻ and H⁺ to form I2 and H2O, and part (b)(ii) asks to calculate that 7.50 × 10⁻⁴ mol of I2 is formed from 25.0 cm³ of 0.0100 mol dm⁻³ KIO3. Part (c) outlines adding 10.0 cm³ of the tablet solution to the iodine and titrating the remaining unreacted iodine with 0.100 mol dm⁻³ Na2S2O3, giving a mean titre of 14.40 cm³. Students must name the indicator and its end-point colour change in (c)(i), and deduce by calculation whether the label claim of 6% vitamin C by mass is correct in (c)(ii).
Question text

10 Vitamin C has the molecular formula C6H8O6.

The label on a bottle of vitamin C tablets stated that a 2.50g tablet contained 6%

of vitamin C by mass. The tablet was analysed to check the accuracy of the label.

The procedure involved a series of steps.

(a) Step 1 Dissolving the tablet.

A 2.50g vitamin C tablet was crushed and dissolved to make an aqueous solution

of volume 250.0 cm3 .

Describe how to make this solution from the crushed tablet.

(3)

(b) Step 2 Producing a known amount of iodine.

Iodine was produced by reacting 25.0 cm3 of 0.0100 mol dm–3 potassium iodate

with excess potassium iodide and hydrochloric acid in a conical flask.

(i) Complete the ionic equation for the formation of the iodine from 1 mol of

IO− ions.

(1)

IO− + I− + H+ → 3I +

3 … 2 …

(ii) Show, by calculation, that 7.50 × 10−4 moles of iodine were produced in

the flask.

(2)

*P76895A02528*

(c) Step 3 Titrating with sodium thiosulfate solution.

10.0 cm3 of the vitamin C tablet solution from Step 1 was added to the conical

flask from Step 2 to react with the iodine produced, as shown in the equation.

C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2HI(aq)

The unreacted iodine in the conical flask was titrated with a solution of

0.100 mol dm−3 sodium thiosulfate, Na S2O (aq).

The mean titre was 14.40 cm3 .

2S O32–(aq) + I (aq) → 2I–(aq) + S O2−(aq)

22 4 6

(i) State the indicator used in this titration, giving the colour change that would

be observed at the end-point.

(2)

… (ii)Deduce, by calculation, whether the label on the bottle of vitamin C tablets

is correct.

26 (6)

*P76895A02628*

(Total for Question 10 = 14 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 10 totaling 14 marks. Part (a) awards 3 marks for: dissolving solid in deionised water in a beaker and transferring to a volumetric flask; rinsing beaker and transferring washings; and making up to the calibration mark and inverting to mix. Part (b)(i) awards 1 mark for balancing IO3⁻ + 5I⁻ + 6H⁺ → 3I2 + 3H2O. Part (b)(ii) awards 2 marks for finding moles of iodate (2.5 × 10⁻⁴) and multiplying by 3 to reach 7.50 × 10⁻⁴ mol. Part (c)(i) awards 2 marks for starch indicator changing from blue-black to colourless. Part (c)(ii) awards 6 marks for finding moles of thiosulfate (1.44 × 10⁻³), unreacted iodine (7.2 × 10⁻⁴), reacting vitamin C in 10 cm³ (3.00 × 10⁻⁵), scaling up to 250 cm³ (7.5 × 10⁻⁴ mol), mass of ascorbic acid (0.132 g), and concluding the label is incorrect as 0.132 g is less than 0.15 g (or 5.28% is less than 6%).

Question

Acceptable Answer Additional Guidance Mark

Number

10(a) An answer that makes reference to the following points: Steps have to be in a logical order. (3)

• (dissolve ) in deionised water (in a beaker) (1) Allow distilled for deionised

and Allow direct transfer of the solid to the volumetric

transfer to a volumetric flask flask

Ignore use of pestle and mortar

Comment allow this mark as long as deionised

/distilled water is seen in the response

• (wash the beaker and) transfer the washings (1)

• make up to the mark and invert / mix (thoroughly) (1) Allow any indication of mixing eg shaking /

stirring / etc

Question

Acceptable Answer Additional Guidance Mark

Number

10(b)(i) An answer that makes reference to the following point: (1)

• IO − + 5I− + 6H+ → + 3I + 3H O

32 2

Question

Acceptable Answer Additional Guidance Mark

Number

10(b)(ii) Example of calculation (2)

• moles potassium iodate (1) 0.01 × 0.025 = 2.5 × 10−4

(1) 3 × 2.5 × 10-4 = 7.5 × 10−4

• moles iodine generated

Allow calculation shown in one step.

No TE on incorrect values

Allow answers not in standard form

Question

Acceptable Answer Additional Guidance Mark

Number

10(c)(i) An answer that makes reference to the following points: (2)

• starch (1)

• blue-black to colourless (1) Allow blue or black to colourless

Do not award light blue

M2 depends on M1

Question

Acceptable Answer Additional Guidance Mark

Number

10(c)(ii) Example of calculation (6)

• moles thiosulfate used in titration (1) = 14.40 × 0.100 ÷ 1000 = 1.44 × 10−3 / 0.00144

(1) = 1.44 × 10−3 = 7.2 × 10−4 / 0.00072

• moles unreacted iodine in flask

• moles of iodine that reacted with 10.0 cm3 vitamin C (1) = 7.5 × 10−4 − 7.2 × 10−4

= 3.00 × 10−5 / 0.00003

tablet solution

and

(therefore) moles of ascorbic acid in 10.0 cm3

vitamin C tablet solution

3 (1) = 3.00 × 10−5 × 25 = 7.5 × 10−4 / 0.00075

• moles of ascorbic acid in 250 cm solution (1 tablet)

(1) = 7.5 × 10−4 × 176 = 0.132 (g)

• mass of ascorbic acid in 1 tablet

• expected mass of ascorbic acid in 1 tablet (1) Expected mass of ascorbic acid in tablet =

and 6 / 100 × 2.5 = 0.15 g (which is greater than 0.132 g)

the label is wrong (as there is too little vitamin C in

each tablet)

or

percentage of ascorbic acid in 1 tablet Percentage = 0.132 ÷ 2.50 × 100 = 5.28%

and

the label is wrong (as there is too little vitamin C in Calculations for MP4 and MP5 can be done in

each tablet) either order

Correct answer without working scores 6

TE at all stages

(Total for Question 10 = 14 marks)

TOTAL FOR PAPER = 90 MARKS

How to answer it

Analysis of Vitamin C Tablets by Back Titration

📌 What this question tests

This question assesses practical techniques and quantitative analytical chemistry involving redox systems:

  • Standard Solution Preparation: Standard laboratory procedure for transferring a solid, washing, making up to volume, and homogenising in a volumetric flask.
  • Redox Chemistry & Balancing: Balancing ionic equations for the reduction of iodate(V) ions by iodide in acidic conditions.
  • Stoichiometry & In-situ Generation: Calculating moles of generated reagent (I₂) from a primary standard.
  • Redox Indicators: Correct choice of indicator (starch) and observation of a sharp end-point in iodine/thiosulfate titrations.
  • Multi-step Back Titration Calculations: Working backwards from titre to find unreacted reagent, reacting reagent, scaling up from an aliquot to the bulk solution, calculating experimental mass/percentage, and drawing an evaluative conclusion against a commercial claim.

Part (a) Making the Standard Solution

Core Practical 1 / Volumetric Preparation Skills [3 Marks]

✅ Expected Mark Scheme Points

  1. Dissolve & Transfer: Dissolve the crushed tablet in deionised (or distilled) water in a beaker, then transfer the solution into a 250.0 cm³ volumetric flask.
  2. Rinse: Rinse the beaker, stirring rod, and funnel with deionised water, and transfer all washings into the volumetric flask.
  3. Make up & Invert: Make up to the graduation mark with deionised water until the bottom of the meniscus is on the line, then stopper and invert/shake repeatedly to mix thoroughly.

🧠 Exam Technique & Examiner Commentary

  • Chronological Order: Steps must follow a clear, logical sequence. You cannot say "invert" before "making up to the mark".
  • Name the Glassware: Always specify a volumetric flask (not just "a flask" or "conical flask").
  • Water Specification: Always explicitly mention deionised or distilled water rather than just "water".

❌ Common Errors

  • Dissolving the tablet directly in the narrow neck of the volumetric flask without stirring.
  • Forgetting to transfer the washings from the beaker, which causes a loss of solute and systematic error.
  • Omitting the final mixing/inversion step (failing to homogenise the solution).
Mark Breakdown:
• [1 Mark]: Dissolves in deionised/distilled water in a beaker & transfers to volumetric flask.
• [1 Mark]: Washes beaker and transfers washings.
• [1 Mark]: Makes up to the mark and inverts/mixes thoroughly.

Part (b) Production of Iodine

Balancing Redox Equations & Mole Calculations [3 Marks total]

✅ Correct Balanced Equation (b)(i) [1 Mark]

IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O

Balancing atoms & charges:
Left: -1 + 5(-1) + 6(+1) = 0 charge; 1 + 5 = 6 iodine atoms; 3 oxygens; 6 hydrogens.
Right: 0 charge; 3 × 2 = 6 iodine atoms; 3 oxygens; 6 hydrogens.

📐 Calculation (b)(ii) [2 Marks]

Goal: Show that 7.50 × 10⁻⁴ mol of I₂ is produced.

  1. Step 1: Moles of IO₃⁻ used
    n(IO₃⁻) = c × V
    n(IO₃⁻) = 0.0100 mol dm⁻³ × (25.0 / 1000) dm³ = 2.50 × 10⁻⁴ mol [1 Mark]
  2. Step 2: Use stoichiometry from (b)(i)
    1 mol IO₃⁻ produces 3 mol I₂.
    n(I₂) = 3 × 2.50 × 10⁻⁴ mol = 7.50 × 10⁻⁴ mol [1 Mark]

❌ Common Pitfalls in (b)

  • Forgetting 1 : 3 ratio: Stopping at 2.50 × 10⁻⁴ mol without multiplying by 3.
  • Unit Conversion: Forgetting to divide 25.0 cm³ by 1000 to convert to dm³.

Part (c)(i) Titration Indicator & End-Point

Iodine / Thiosulfate Titration Visualisation [2 Marks]

✅ Correct Answer

  • Indicator: Starch [1 Mark]
  • End-point colour change: Blue-black to colourless [1 Mark]

💡 Key Knowledge & Examiner Notes

  • The mark scheme strictly accepts blue-black (or blue, or black) to colourless.
  • Do NOT write "clear": "Clear" is not a colour in chemistry; state colourless.
  • "Light blue" is not awarded. The solution must decolourise completely when all unreacted iodine has reacted.
  • Starch is added only when the iodine solution fades to a pale straw-yellow colour to ensure reversible binding.

Part (c)(ii) Quantitative Back Titration Deduction

Multi-Step Quantitative Deduction & Evaluation [6 Marks]

📐 Full Step-by-Step Solution

Step 1: Calculate moles of sodium thiosulfate used in titration

n(S₂O₃²⁻) = c × V = 0.100 mol dm⁻³ × (14.40 / 1000) dm³ = 1.44 × 10⁻³ mol

Mark 1 awarded for finding moles of thiosulfate (1.44 × 10⁻³ mol or 0.00144 mol).

Step 2: Calculate moles of unreacted iodine remaining in flask

From equation: 2S₂O₃²⁻ + I₂ → 2I⁻ + S₄O₆²⁻ (Ratio is 2 : 1)

n(unreacted I₂) = 1.44 × 10⁻³ / 2 = 7.20 × 10⁻⁴ mol

Mark 2 awarded for dividing by 2 to get unreacted iodine (7.20 × 10⁻⁴ mol).

Step 3: Calculate moles of iodine that reacted with Vitamin C (in 10.0 cm³ aliquot)

n(I₂ reacted) = n(initial I₂) - n(unreacted I₂)

n(I₂ reacted) = 7.50 × 10⁻⁴ - 7.20 × 10⁻⁴ = 3.00 × 10⁻⁵ mol

From equation: C₆H₈O₆ + I₂ → C₆H₆O₆ + 2HI (Ratio is 1 : 1)

n(vitamin C in 10.0 cm³) = 3.00 × 10⁻⁵ mol

Mark 3 awarded for subtracting from initial moles (7.50 × 10⁻⁴) to find reacting moles of vitamin C in 10.0 cm³.

Step 4: Scale up to the full 250.0 cm³ volumetric solution (entire tablet)

Scaling factor = 250.0 cm³ / 10.0 cm³ = 25

n(vitamin C in 250.0 cm³) = 3.00 × 10⁻⁵ × 25 = 7.50 × 10⁻⁴ mol

Mark 4 awarded for multiplying by 25 to find total moles in the tablet (7.50 × 10⁻⁴ mol).

Step 5: Calculate mass of Vitamin C (C₆H₈O₆) in the tablet

Mᵣ(C₆H₈O₆) = (6 × 12.0) + (8 × 1.0) + (6 × 16.0) = 72.0 + 8.0 + 96.0 = 176.0 g mol⁻¹

mass = n × Mᵣ = 7.50 × 10⁻⁴ mol × 176.0 g mol⁻¹ = 0.132 g

Mark 5 awarded for multiplying by molar mass to get experimental mass (0.132 g).

Step 6: Evaluate claim and make deduction

Route A: Compare Masses
• Expected mass from label (6% of 2.50 g):
2.50 × (6 / 100) = 0.150 g
• Comparison: 0.132 g < 0.150 g
• Conclusion: The label is incorrect (contains less vitamin C than claimed).
Route B: Compare Percentages
• Experimental percentage in tablet:
(0.132 / 2.50) × 100 = 5.28%
• Comparison: 5.28% < 6.00%
• Conclusion: The label is incorrect (tablet is only 5.28% vitamin C).
Mark 6 awarded for expected mass/percentage calculation AND valid conclusion that label is incorrect.

❌ Common Calculation Traps

  • Skipping the 250/10 scaling: Forgetting that only an aliquot of 10.0 cm³ was titrated out of the 250.0 cm³ made up in Step 1.
  • Mixing up ratios: S₂O₃²⁻ to I₂ is 2 : 1, whereas I₂ to C₆H₈O₆ is 1 : 1. Students often flip or forget the 2:1 ratio.
  • Stopping without a conclusion: Calculating 0.132 g or 5.28% but omitting the explicit deduction that "the label is incorrect / contains less than 6%" loses Mark 6.

🧠 Top Tip for Back Titrations

Always draw a mini-flowchart showing where reagents go:

Total I₂ generated (7.50 × 10⁻⁴ mol)
├── Part reacts with Vit C (aliquot)
└── Part remains unreacted (titrated with S₂O₃²⁻)

Recognising that Total = Reacted + Excess prevents arithmetic confusion every time!

Topics

Physical Chemistry · Core Practicals · Topic 3: Redox I · Topic 5: Formulae, Equations and Amounts of Substance · Core Practical 2: Preparation of a standard solution from a solid acid

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.