OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 18
12 marks · Medium difficulty · Structured Questions
Determine the percentage by mass of copper in an ore using a titration method involving copper(II) ions, potassium iodide, and sodium thiosulphate.
Practise this questionQuestion
Question text
18 A student carries out an experiment to determine the percentage by mass of copper in an ore
containing copper in its +2 oxidation state.
The student is provided with a sample of the copper ore, 1 mol dm–3 potassium iodide, KI(aq), and
0.0200 mol dm–3 sodium thiosulfate, Na S O .
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The student’s method is outlined below.
Step 1 Add an excess of warm nitric acid to 2.50 g of the ore.
The copper(II) compounds in the ore react, forming aqueous copper(II) nitrate.
Step 2 Filter the mixture to remove the unreacted rock. Neutralise the filtrate.
Step 3 Add an excess of aqueous potassium iodide, KI(aq).
A precipitate of copper(I) iodide and a solution of iodine, I2(aq), forms.
Step 4 Titrate the mixture from Step 3 using 0.0200 mol dm–3 sodium thiosulfate, Na S O in
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the burette.
I (aq) + 2S O 2–(aq) → 2I–(aq) + S O 2–(aq)
22 3 4 6
26.55 cm3 of 0.0200 mol dm–3 Na S O are required to reach the end point.
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(a) In Step 1, the student observed that bubbles of gas were produced.
Suggest the formula of the copper(II) compound which reacted with HNO3 to form the gas,
and write a full equation for the reaction.
Formula: …
Equation: … [2]
(b) Write an ionic equation, including state symbols, for the reaction in Step 3.
… [1]
(c) Suggest a suitable indicator for this titration and state the colour change at the end point in
Step 4.
Indicator: …
Colour from … to … [1]
(d) Determine the percentage, by mass, of copper in the copper ore.
Give your answer to an appropriate number of significant figures.
percentage = … % [4]
(e) Explain whether the calculated percentage by mass of copper would be higher, lower or the
same if the following changes were made to the method.
(i) The potassium iodide was not in excess, in Step 3.
… [1]
(ii) The burette readings were read from the top of the meniscus, in Step 4.
… [1]
(f) The student then modifies the method in order to obtain a more accurate value for the
percentage by mass of copper in the ore. The student decides to use 25.00 g of the copper
ore in Step 1.
What further modifications should the student make to produce a more accurate value for the
percentage by mass of copper in the ore?
… [2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
18 (a) Formula: CuCO3 2 AO1.2 IGNORE state symbols
ALLOW formula within equation.
CuCO3 + 2HNO3 → Cu(NO3)2 + CO2 + H2O AO2.6
ALLOW other copper(II) compounds which can
react with nitric acid to form a gas e.g. CuS,
CuSO3 for mark 1, with correct equation for mark
2.
e.g.CuSO3 + 2HNO3 → Cu(NO3)2 + SO2 + H2O
(b) 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I (aq) 1 AO2.6 ALLOW multiples
State symbols are required
(c) starch (solution) 1 AO1.2 ALLOW blue OR black OR purple for colour of
AND mixture
blue-black to colourless
ALLOW blue colour disappears (to colourless)
IGNORE ‘clear’
IGNORE ‘colorimetry
(d) FIRST CHECK THE ANSWER ON ANSWER LINE 4 AO2.8 FULL ANNOTATIONS MUST BE USED
If answer = 1.35 award 4 marks ×5 ------------------------------------------------------
---------------------------------------------------------------------- ALLOW ECF throughout
2– 26.55
n(S2O3 ) = 0.0200 ×
1000
= 5.31 × 10–4 (mol)
n(I ) = 2.655 × 10–4 OR
2+ –4 If 1:2 ratio for I :Cu2+ not used check ratio in b) and
n(Cu )= 5.31 × 10 (mol) 2
allow ECF
m(Cu/Cu2+) in ore = 63.5 x 5.31 × 10–4
= 0.0337….. (g) IGNORE rounding errors after 3 SF
0.0337…. Calculator: 0.0337185
percentage = 2.50 × 100 ALLOW 3 SF (0.0337) up to calculator value
= 1.35 (%) (3SF required)
ECF dependent on the use of a calculated mass of
Cu/Cu2+
AO
element
(e) (i) Lower AND smaller titre 1 AO3.4 ALLOW less I produced / less Cu2+ reacts
(ii) The same AND burette measures by difference 1 AO3.4 ALLOW AW
(f) Any two of the following: 2 AO3.4
x 2
Make up a (standard solution) from Step 2 to a stated
volume (e.g. 250 cm3)
OR
Repeat titrations
AND
Take mean of concordant/closest titres/ identify
anomalies
OR
lower [S O ]2- to increase titre volume (to reduce the
percentage error).
OR
higher [S O ]2- so not to refill the burette.
OR
Use a 3 dec place balance (to reduce the percentage
error).
Total 12
How to answer it
Analysis of Copper Ore by Redox Titration
What this question tests
This multi-step synoptic problem tests your mastery of redox titrations, writing ionic equations, mole calculations involving reacting ratios, evaluating experimental errors, and suggesting practical improvements to reduce percentage uncertainties.
Identifying the Copper(II) Compound and Equation
✅ Correct Answer
Formula: CuCO₃ (or copper(II) carbonate)
Equation: CuCO₃ + 2HNO₃ → Cu(NO₃)₂ + CO₂ + H₂O
💡 Key Knowledge
- Observation of "bubbles" or a gas with acid immediately indicates a carbonate reacting.
- Metal carbonates react with acids to form a salt, carbon dioxide gas, and water.
Ionic Equation for Iodide-Copper(II) Reaction
✅ Correct Answer
2Cu²⁺(aq) + 4I⁻(aq) → 2CuI(s) + I₂(aq)
❌ Common Errors
Forgetting state symbols when explicitly requested, or failing to balance the electrons/charges resulting in incorrect stoichiometry (remember Cu²⁺ is reduced to Cu⁺ in copper(I) iodide).
Choosing an Indicator and Colour Change
✅ Correct Answer
Indicator: Starch (solution)
Colour change: Blue-black to colourless
🧠 Exam Technique
Starch is the specific indicator used for titrations involving iodine ( I₂ ) and thiosulfate ( S₂O₃²⁻ ). Always state the colour *from* and *to* in the correct chronological order of the titration.
Calculating Percentage by Mass of Copper
📐 Step-by-Step Calculation
- Moles of thiosulfate used:
n(S₂O₃²⁻) = 0.0200 × (26.55 / 1000) = 5.31 × 10⁻⁴ mol - Moles of iodine and copper(II):
From equation I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ , moles of I₂ = half of thiosulfate = 2.655 × 10⁻⁴ mol .
From part (b), 2 moles of Cu²⁺ produce 1 mole of I₂ , meaning n(Cu²⁺) = n(S₂O₃²⁻) = 5.31 × 10⁻⁴ mol . - Mass of copper:
Mass = moles × Ar = 5.31 × 10⁻⁴ × 63.5 = 0.0337185 g - Percentage by mass:
Percentage = (0.0337185 / 2.50) × 100 = 1.3487% → round to 3 significant figures: 1.35%
❌ Calculation Traps
Using the wrong reacting ratio between thiosulfate and iodine is the most common place students lose marks. Always write out or check mole ratios carefully!
📐 Significant Figures
Ensure your final answer matches the lowest precision of the data given in the question (3 significant figures, matching 2.50 g and 0.0200 mol dm⁻³ ).
Evaluating Experimental Modifications
(i) Potassium iodide not in excess
Answer: Lower percentage AND smaller titre.
Reasoning: If KI is limiting, not all Cu²⁺ reacts to form I₂ , resulting in less I₂ titrated and a smaller titre volume.
(ii) Burette read from top of meniscus
Answer: The same percentage AND burette measures by difference.
Reasoning: Titrations rely on titre differences ( Final - Initial ). A systematic error in reading both the start and end point consistently cancels out.
Improving Accuracy and Reducing Uncertainty
✅ Acceptable Answers (Any two)
- Make up a standard solution from Step 2 to a stated volume (e.g., 250 cm³ ) and titrate aliquots.
- Repeat titrations to obtain concordant titres and eliminate anomalies.
- Use a lower concentration of S₂O₃²⁻ to increase the titre volume and reduce percentage error.
- Use a balance weighing to 3 decimal places to decrease percentage mass uncertainty.
🧠 Examiner Insight
Top-level responses focus on either reducing percentage measurement uncertainty (using larger titres or more precise glassware/balances) or improving reliability through titration replication and averaging.
Topics
Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · Practical Activity Groups · PAG 2: Acid-base titration · 5.3 Transition elements · 2.1 Atoms and reactions
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.