Edexcel A-Level Chemistry Paper 3, June 2024: Question 9
15 marks · Medium difficulty · Calculations
Determine the concentration of tartaric acid and sulfur dioxide in wine using titration techniques, safety precautions, and redox/acid-base calculations.
Practise this questionQuestion
Question text
9 Acidity in wine is mainly due to the dicarboxylic acid tartaric acid, C4H6O6.
Sulfur dioxide is added to wine as an antioxidant, to prevent the colour and flavour of
the wine from deteriorating. It also inhibits the growth of bacteria.
The concentration of tartaric acid and the concentration of sulfur dioxide present in
wine can both be determined by titrations.
(a) The procedure to find the total amount of tartaric acid in a white wine is shown.
Step 1 Pipette a 20.0 cm3 sample of the wine into a conical flask.
Step 2 Add anti-bumping granules to the flask, and then place it on a
preheated hot plate.
Step 3 Boil the sample to remove any dissolved gases and allow it to cool.
Step 4 Use 0.100 mol dm–3 sodium hydroxide solution, NaOH(aq), in a burette to
titrate the sample using a suitable indicator.
(i) State a precaution you would take in carrying out Step 3.
Justify your answer.
(2)
(ii) Explain what effect the omission of Step 3 would have on the titre.
(2)
(iii) A mean titre of 20.60 cm3 was obtained.
Calculate the concentration, in g dm–3, of tartaric acid in the white wine.
C4H6O6 + 2NaOH → C4H4O6Na2 + 2H2O
(3)
*P74455A02332*
(iv) A student carrying out the experiment forgot to remove an air bubble from
the tip of the burette before carrying out a titration. During the titration, the
air bubble escaped.
Explain the effect this mistake would have on the value of this titre.
(2)
(b) A procedure to find the amount of sulfur dioxide, SO2, in a wine is shown.
Step 1 50.0 cm3 of a white wine is added to a conical flask containing 5 cm3 of
dilute sulfuric acid and a few drops of starch indicator.
Step 2 A spatula measure of sodium hydrogencarbonate is then added to reduce
the loss of sulfur dioxide from the flask.
Step 3 The contents of the flask are titrated rapidly using 0.0100 mol dm–3 iodine
solution, I2(aq).
The equation for the reaction that takes place is shown.
SO (aq) + I (aq) + 2H O(l) → 2I–(aq) + 4H+(aq) + SO2–(aq)
22 2 4
(i) A student carrying out the experiment obtained a titre of 11.80 cm3.
The maximum permitted level of sulfur dioxide in white wine in the UK is
200 mg dm–3.
24 Show by calculation that the concentration of sulfur dioxide in the white wine
is below the permitted maximum.*P74455A02432*
(4)
(ii) Explain how the addition of sodium hydrogencarbonate in Step 2 reduces the
loss of sulfur dioxide from the flask.
(2)
(Total for Question 9 = 15 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
9(a)(i) An answer that makes reference to the following points: (2)
• carry out in a fume cupboard (1) Allow in a well-ventilated laboratory
Ignore references to handling safely etc
• SO2 is toxic / poisonous (1) Allow SO2 is corrosive
Allow reference to SO2 issues for breathing difficulties
e.g. asthma
Allow reference to gases/fumes for SO2
Ignore acidic/harmful/irritant
Question
Answer Additional Guidance Mark
Number
9(a)(ii) An explanation that makes reference to the following points: (2)
• SO2 form acids when dissolved (1) Allow SO2 is acidic
Allow SO2 reacts with the NaOH
Ignore CO2
• so the titre value would be greater (than
expected) / results would (appear to) show more M2 is dependent upon M1 or a near miss such as the titre
(tartaric) acid (than was present in the wine) (1) value is increased because of greater acidity or the
reaction of dissolved gases with NaOH
Question
Answer Additional Guidance Mark
Number
9(a)(iii) A calculation that makes reference to the following points: (3)
• calculation of amount of NaOH (20.6 ÷ 1000) × 0.100 = 2.06 × 10–3 (mol)
and
amount of tartaric acid (1) = 2.06 × 10–3 ÷ 2 = 1.03 × 10–3 (mol)
–3 (1) 1.03 × 10–3 ÷ (20 ÷ 1000) = 0.0515 (mol dm–3)
• calculation of amount of tartaric acid in mol dm
–3 (1) = 0.0515 × 150 = 7.725 (g dm–3)
• calculation of amount of tartaric acid in g dm
Accept step 3 carried out before step 2
Ignore intermediate units even if incorrect
Ignore SF except 1SF
TE throughout
Question
Answer Additional Guidance Mark
Number
9(a)(iv) An explanation that makes reference to the following points: (2)
• the titre value includes the volume of the air (1) Allow some alkali/solution is used to fill the
bubble (as well as sodium hydroxide solution) air bubble/ burette tip/burette jet
• so the titre value would be greater (than expected) (1) M2 is dependent upon M1 or a near miss
Question
Answer Additional Guidance Mark
Number
9(b)(i) An explanation that makes reference to the following points: (4)
• calculation of amount of iodine (1) (11.80 ÷ 1000) × 0.01 = 1.18 × 10–4 / 0.000118 (mol)
–3 (1) 1.18 × 10–4 ÷ (50 ÷ 1000) = 2.36 × 10–3 / 0.00236 (mol dm–3)
• calculation of concentration of SO2 in mol dm
–3 (1) 2.36 × 10–3 × 64.1 = 0.15128 (g dm–3)
• calculation of concentration of SO2 in g dm
–3 = 0.15128 × 1000 = 151 (mg dm–3)
• calculation of concentration of SO2 in mg dm
and
comment regarding allowable level (1) so below maximum permitted level
Allow just ‘so ok/suitable’ or 151 < 200 for comment
Accept calculations in steps 2 to 4 in any order
Accept calculation carried out in reverse order, i.e. from
permitted mass to moles
Ignore intermediate units
Final answer without working scores (4)
Allow use of 64 for Mr of SO2
Allow final answer in g dm–3 if permitted maximum
converted to g dm–3
Ignore SF except 1SF
TE throughout provided that the numbers are carried forward
Question
Answer Additional Guidance Mark
Number
9(b)(ii) An explanation that makes reference to the following points: (2)
• to produce CO2 (1)
• which forms a blanket / layer (on surface of solution) (1) Allow CO2 is denser / heavier than air (so stops escape of
SO2)
Allow SO2 cannot get past the (blanket of ) CO2
(Total for Question 9 = 15 marks)
How to answer it
Analysis of Acidity and Sulfur Dioxide in Wine
What this question tests
This multi-part A-Level Chemistry question assesses practical titration techniques, safety precautions involving toxic gases (sulfur dioxide), effects of procedural errors on titres, stoichiometric calculations (moles, concentration in g dm⁻³, conversion to mg dm⁻³), and gas equilibria/blanket effects.
Precaution and Justification for Boiling Step
✅ Correct Answer
- Precaution: Carry out the procedure in a fume cupboard. (1 mark)
- Justification: Sulfur dioxide (SO₂) is toxic, poisonous, or corrosive, or causes breathing difficulties. (1 mark)
❌ Common Errors
Students often lose marks by writing vague statements like "wear safety glasses" or "handle carefully". The precaution must directly relate to the hazard of releasing dissolved toxic sulfur dioxide gas during heating.
Effect of Omitting Step 3 (Boiling)
✅ Correct Answer
- Sulfur dioxide is acidic and reacts with the sodium hydroxide solution. (1 mark)
- Therefore, the titre value would be greater than expected. (1 mark)
💡 Key Knowledge
Dissolved gases like sulfur dioxide form acidic solutions in water. If not boiled off, they will neutralise some of the NaOH(aq) alongside the tartaric acid, inflating the required titre.
Calculation of Tartaric Acid Concentration
📐 Step-by-Step Calculation
- Moles of NaOH used:
(20.60 ÷ 1000) × 0.100 = 2.06 × 10⁻³ mol - Moles of tartaric acid (C₄H₆O₆) in 20.0 cm³:
Using stoichiometry ratio 1:2 from C₄H₆O₆ + 2NaOH → C₄H₆O₆Na₂ + 2H₂O
2.06 × 10⁻³ ÷ 2 = 1.03 × 10⁻³ mol - Concentration in mol dm⁻³:
1.03 × 10⁻³ ÷ (20.0 ÷ 1000) = 0.0515 mol dm⁻³ - Concentration in g dm⁻³:
Molar mass of C₄H₆O₆ = (4×12.0) + (6×1.0) + (6×16.0) = 150 g mol⁻¹
0.0515 × 150 = 7.725 g dm⁻³
Burette Air Bubble Error
✅ Correct Answer
- The titre value includes the volume of the air bubble that filled during the titration. (1 mark)
- Therefore, the recorded titre value would be greater than expected. (1 mark)
🧠 Exam Technique
Always consider what occupies the space in the burette tip. If an air bubble fills up with liquid *during* the titration, extra liquid has left the burette that wasn't accounted for by the initial scale reading, making the final titre artificially large.
Sulfur Dioxide Titration & Limit Check
📐 Step-by-Step Calculation
- Calculate moles of iodine (I₂):
(11.80 ÷ 1000) × 0.0100 = 1.18 × 10⁻⁴ mol - Calculate concentration of SO₂ in mol dm⁻³:
From equation, 1 mol SO₂ reacts with 1 mol I₂. Therefore moles SO₂ = 1.18 × 10⁻⁴ mol in 50.0 cm³.
1.18 × 10⁻⁴ ÷ (50.0 ÷ 1000) = 2.36 × 10⁻³ mol dm⁻³ - Calculate concentration of SO₂ in g dm⁻³:
Molar mass of SO₂ = 32.1 + (2×16.0) = 64.1 g mol⁻¹
2.36 × 10⁻³ × 64.1 = 0.15128 g dm⁻³ - Convert to mg dm⁻³ and comment:
0.15128 × 1000 = 151 mg dm⁻³
Since 151 mg dm⁻³ is less than the permitted 200 mg dm⁻³, it is below the permitted maximum. (1 mark)
Role of Sodium Hydrogencarbonate
✅ Correct Answer
- Reacts to produce carbon dioxide gas (CO₂). (1 mark)
- Carbon dioxide forms a blanket or layer on the surface of the solution, preventing the escape of sulfur dioxide gas. (1 mark)
💡 Key Knowledge
CO₂ is denser than air, creating a physical protective barrier above the liquid phase that stops volatile SO₂ gas from escaping into the atmosphere prior to titration.
Topics
Physical Chemistry · Core Practicals · Core Practical 3: Find the concentration of a solution of hydrochloric acid · Topic 5: Formulae, Equations and Amounts of Substance
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.