Edexcel A-Level Chemistry Paper 3, June 2022: Question 9
14 marks · Hard difficulty · Practical Techniques and Data Analysis
Analyze the acid-base titration of citric acid in pineapple juice, redox titration of ascorbic acid using potassium iodate, and the identification of interfering compounds containing carboxylic acid groups.
Practise this questionQuestion
Question text
9 Pineapple juice contains the weak acids citric acid (C6H8O7) and ascorbic acid (C6H8O6).
The amount of each compound in a sample of 150 cm3 of pineapple juice can be
determined by titration.
(a) Experiment 1 is designed to determine the total amount of acid.
10.0 cm3 samples of pineapple juice are transferred to separate conical flasks and
titrated with a solution of sodium hydroxide of known concentration.
The total amount of acid in the 150 cm3 sample of pineapple juice is
8.00 × 10–3 mol.
(i) Give a reason why methyl orange would not be a suitable indicator to use in
this titration.
(1)
(ii) A student did not notice an air bubble in the tip of the burette before
carrying out one of their accurate titrations. During this titration, the air
bubble escaped.
Explain the effect this mistake would have on the value of this titre.
(2)
(b) Experiment 2 is carried out to determine the amount of ascorbic acid (C6H8O6) in
the pineapple juice.
An outline procedure for this experiment is given.
Step 1 5.00 cm3 of the pineapple juice is added to a conical flask.
Step 2 Deionised water, a small amount of HCl(aq), a few crystals of
potassium iodide, KI, and 3 drops of starch solution are also added to
the flask.
Step 3 The contents of the flask are swirled to ensure the KI dissolves fully.
Step 4 The resultant mixture is titrated with a solution of
potassium iodate(V), KIO (aq), of concentration 0.00100 mol dm–3.
The reactions that take place are
IO–(aq) + 5I–(aq) + 6H+(aq) → 3I (aq) + 3H O(l)
32 2
24 C H O6(aq) + I (aq) → C H O (aq) + 2H+(aq) + 2I–(aq)
*P67095A02436*682666
Only the ascorbic acid reacts with the iodine.
(i) The end-point of the titration is when the starch changes colour.
Explain how this occurs, including the colour change.
(3)
(ii) The total amount of acid in the 150 cm3 sample is 8.00 × 10–3 mol.
The mean titre in Experiment 2 using 5.00 cm3 of pineapple juice is 9.50 cm3.
Calculate the mass of citric acid in the 150 cm3 sample.
(5)
*P67095A02536*
(c) While doing background research for the experiment, a student found that three
other compounds, D, E and F, are often present in pineapple juice.
O
H C O CH3
HO
OH
HO O O OH
Compound D Compound E
CH CH H3C
H C CH 3 3
CH CH H3C CH3
CH 3 3
Compound F
Predict which one of these compounds is most likely to affect the result of
Experiment 1 and hence predict the effect on the mass of citric acid calculated
in (b)(ii).
Justify your answer.
(3)
(Total for Question 9 = 14 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
9(a)(i) An answer that makes reference to one of the following points: (1)
• the colour of the pineapple juice masks the colour change Allow methyl orange is a similar colour to
or pineapple juice
methyl orange only works with a strong acid Accept methyl orange cannot be used with a weak
or acid (and strong alkali)
methyl orange does not change colour in the vertical section Allow the pH range / 3.2-4.4 / pKin of methyl
of the titration curve orange is below the equivalence point / too low
Allow the colour change would occur before the
equivalence point / is not over the equivalence
point
Allow the pH at the equivalence point is not in the
pH range of methyl orange
Allow end point for equivalence point
Ignore just ‘no colour change observed’
Ignore just ‘end point is not accurate’
Question
Answer Additional Guidance Mark
Number
9(a)(ii) An explanation that makes reference to the following points: (2)
• the titre value would be greater (than expected) (1)
• as the titre value includes the volume of the air bubble (as well as M2 conditional on M1 scored
sodium hydroxide solution) (1) Allow some alkali / solution is used to fill
the air bubble / jet
Allow there is less sodium hydroxide in
the burette than expected
Question
Answer Additional Guidance Mark
Number
9(b)(i) An explanation that makes reference to the following points: (3)
• (at the end point) all ascorbic acid is used up so the iodine is no longer
reduced (to iodide ions)
or
ascorbic acid reacts with the iodine until all the (ascorbic) acid is used
up (1)
• the (slight excess) iodine reacts / forms complex with the starch (1) Stand alone
Allow starch in the presence of iodine
Do not award starch and iodide ions
• (changing from yellow) to a blue/black colour (1) Stand alone
Allow just black or just (dark) blue
Ignore initial colour of solution
Do not award blue/black to colourless
Question
Answer Additional Guidance Mark
Number
9(b)(ii) Example of calculation (5)
• calculation of amount of IO − (aq) (1) = (9.50/1000) x 0.00100 = 9.50 x 10–6 (mol)
= 9.50 x 10–6 x 3 = 2.85 x 10–5 (mol)
• calculation of amount of iodine / ascorbic acid in
5.00 cm3 sample (1) TE on M1
= 2.85 x 10–5 x 30 = 8.55 x 10–4 (mol)
• calculation of amount of ascorbic acid in 150.0
cm3 sample (1) TE on M2
3 = 8.00 x 10–3 – 8.55 x 10–4 = 7.145 x 10–3 (mol)
• calculation of amount of citric acid in 150.0 cm
sample (1) TE on M3
= 7.145 x 10–3 x 192 = 1.37184 g = 1.37 (g)
• calculation of mass of citric acid in 150.0 cm3
sample (1) TE on M4
Ignore SF except 1 SF
Correct answer with some or no working scores (5)
Question
Answer Additional Guidance Mark
Number
9(c) An explanation that makes reference to the following points: (3)
• compound E Allow Compound E is a (di)carboxylic acid
and Ignore reference to OH group
as it has (two) COOH / (carboxylic) acid group(s) (1) Do not award carbonyl group(s)
• these / this will (also) react with the NaOH / in the titration (in Do not award if OH group reacts with
Experiment 1) (1) NaOH
• (the titre will be greater in Experiment 1 so suggests a greater total Conditional on compound E selected
amount of acid)
so the final mass of citric acid calculated will be greater (than the
true amount)
or
the total amount of acid (calculated from the titration) includes
citric acid and E so the actual mass of citric acid is less (than
calculated in (b)(ii) (1)
(Total for Question 9 = 14 marks)
How to answer it
Analysis of Acids in Pineapple Juice
What this question tests
This multi-step synoptic question assesses core practical and analytical competencies including acid-base titrations, redox titrations (iodine-iodate chemistry), indicator selection criteria, experimental error analysis, multi-stage mole calculations, and organic functional group reactivity in complex natural mixtures.
Why Methyl Orange is Unsuitable
✅ Correct Answer
The colour of the pineapple juice masks the colour change, OR methyl orange only works with a strong acid (and strong alkali), OR its pH range (3.2–4.4) does not fall within the vertical section of the weak acid–strong base titration curve.
❌ Common Errors
Students often lost this mark by writing vague statements like "no colour change observed" or "the end-point is not accurate" without giving the chemical or physical reason why.
Effect of an Air Bubble in the Burette Tip
✅ Correct Answer
1. The titre value would be greater (than expected).
2. Reason: The titre value includes the volume of the air bubble in addition to the sodium hydroxide solution delivered.
🧠 Exam Technique
Mark 2 is conditional on scoring Mark 1. Always explain what occupies the space where standard solution should be when dealing with faulty burettes or pipettes.
Starch Indicator Behaviour in Iodine Titrations
✅ Correct Answer
1. All ascorbic acid is used up so iodine is no longer reduced to iodide ions.
2. The slight excess of iodine reacts / forms a complex with the starch.
3. Colour changes from yellow to blue/black.
💡 Key Knowledge
In this redox setup, iodine is produced in situ. As long as ascorbic acid is present, iodine is instantly reduced back to colourless iodide ( I⁻ ). The moment ascorbic acid is depleted, excess unreacted iodine complexes with starch to reveal the characteristic dark colour.
Calculating the Mass of Citric Acid
📐 Step-by-Step Calculation
- Moles of IO₃⁻ used:
(9.50 / 1000) × 0.00100 = 9.50 × 10⁻⁶ mol - Moles of I₂ / Ascorbic acid in 5.00 cm³ sample:
From equation 1, 1 mole of IO₃⁻ produces 3 moles of I₂ .
9.50 × 10⁻⁶ × 3 = 2.85 × 10⁻⁵ mol - Moles of ascorbic acid in total 150 cm³ sample:
Scale up from 5.00 cm³ to 150 cm³ (factor of 30):
2.85 × 10⁻⁵ × 30 = 8.55 × 10⁻⁴ mol - Moles of citric acid in 150 cm³ sample:
Total acid minus ascorbic acid = citric acid:
8.00 × 10⁻³ - 8.55 × 10⁻⁴ = 7.145 × 10⁻³ mol - Mass of citric acid ( C₆H₈O₇ , Mr = 192):
7.145 × 10⁻³ × 192 = 1.37184 g = 1.37 g (to 3 sig figs)
Predicting Interference from Juice Impurities
✅ Correct Answer
• Compound E is most likely to affect Experiment 1.
• Justification: E is a di(carboxyic) acid / contains two -COOH groups.
• These carboxylic acid groups will also react with NaOH in the acid-base titration.
• Therefore, the titre in Experiment 1 will be greater, suggesting a greater total acid amount, leading to a falsely elevated calculated mass of citric acid in (b)(ii).
❌ Common Errors
Do not confuse alcohol or ester -OH groups with carboxylic acid groups. Only carboxylic acids act as acidic species capable of neutralising sodium hydroxide under standard titration conditions here.
Topics
Physical Chemistry · Organic Chemistry · Inorganic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 12: Acid-base Equilibria · Topic 3: Redox I · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.