Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 6
20 marks · Hard difficulty · Synoptic Questions
Calculate the percentage purity of malachite, determine the volume of carbon dioxide produced using the ideal gas equation, and explain flame tests and electron transitions.
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Question text
6 Malachite is a green mineral with the formula Cu CO (OH) . It has a molar mass of 221 g mol–1.
23 2
(a) What is the percentage by mass of copper in pure malachite?
(1)
A 40.3%
B 51.4%
C 57.5%
D 67.9%
(b) Describe what you would expect to see when an excess of dilute hydrochloric acid
is added to a sample of pure solid malachite.
(3)
(c) (i) Describe how you would carry out a flame test on a sample of powdered malachite.
(3)
(ii) When the atoms of some elements are heated, they produce a characteristic*P62306A01424*
flame colour. For example, the copper in malachite gives a blue-green colour.
Explain how atoms of different elements can produce different characteristic
flame colours when heated.
(4)
(d) (i) When malachite is heated to approximately 300°C, water, carbon dioxide and
copper(II) oxide are formed.
The equation for this decomposition is
Cu2CO3(OH)2 → 2CuO + CO2 + H2O
Calculate the maximum volume of carbon dioxide that could be produced
when 0.810g of malachite is thermally decomposed.
Assume that the gas is collected at a temperature of 25°C and 101kPa pressure.
Give your answer to an appropriate number of significant figures and state the units.
[The ideal gas equation is pV = nRT. Gas constant (R) = 8.31 J mol–1 K–1]
(5)
*P62306A01524*
(ii) The gas was collected in a gas syringe with a stated accuracy of ± 0.5 cm3.
Calculate the percentage uncertainty in the volume of gas collected.
(1)
(iii) Malachite ore is a mixture of malachite and rock. A 0.810g sample of malachite ore
was thermally decomposed, producing 0.571g of copper(II) oxide.
Calculate the percentage purity of this malachite ore sample.
Give your answer to an appropriate number of significant figures.
(3)
*P62306A01624*
(Total for Question 6 = 20 marks)
Mark scheme
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How to answer it
Analysis and Thermal Decomposition of Malachite
What this question tests
This comprehensive AS Level Chemistry question assesses core physical and inorganic topics: percentage composition, practical observation of acid-base reactions, flame testing techniques, atomic emission spectra explanations, the ideal gas equation (pVT calculations), percentage uncertainty, and percentage purity calculations.
Calculating Empirical Composition
✅ Correct Answer
C (57.5%)
💡 Key Knowledge
Percentage mass = (Total RAM of copper / Molar mass of malachite) × 100
RAM of Cu = 63.5. There are 2 copper atoms: 2 × 63.5 = 127.0. Total molar mass = 221 g mol⁻¹.
(127.0 / 221) × 100 = 57.46% (rounds to 57.5%).
❌ Common Errors
Distractor A (40.3%) calculates the mass percentage of a fragment like CuCO₃(OH) rather than just copper, whilst other options mix up incorrect components from the formula.
Reaction with Hydrochloric Acid
✅ Correct Answers (3 Marks total)
- Solid / malachite / Cu₂CO₃(OH)₂ disappears (allows dissolves)
- Effervescence / fizzing / bubbles (due to CO₂ gas release)
- Green or blue solution produced
🧠 Exam Technique
When an examiner asks what you would see, list distinct sensory observations. Avoid chemical explanations unless requested (e.g., do not just write "carbon dioxide is given off" as it is invisible; write "fizzing/bubbles").
Analytical Techniques & Quantum Transitions
✅ Part (i): Flame Test Procedure (3 Marks)
- Use a platinum or nichrome wire / loop.
- Dip the wire into clean, fresh concentrated hydrochloric acid (HCl).
- Dip the wet wire into the solid sample and place it into a non-luminous (roaring) Bunsen flame.
✅ Part (ii): Flame Colour Theory (4 Marks)
- Electrons absorb energy and move up energy levels / are excited or promoted.
- Electrons return to a lower energy level / ground state.
- Energy is emitted / released from the atom as visible light / flame colour.
- Different elements have different energy gaps, emitting different frequencies/colours.
❌ Common Errors
In part (ii), students frequently lose marks by writing that "atoms" move up energy levels instead of "electrons". Be scientifically precise!
Thermal Decomposition & Gas Volumes
📐 Part (i) Step-by-Step Gas Calculation (5 Marks)
- Find moles of malachite:
0.810 g / 221 g mol⁻¹ = 3.66 × 10⁻³ mol.
From equation, 1 mol Cu₂CO₃(OH)₂ produces 1 mol CO₂, so moles of CO₂ = 3.66 × 10⁻³ mol. - Convert temperature to Kelvin:
25 °C + 273 = 298 K. - Convert pressure to Pascals:
101 kPa = 101,000 Pa. - Rearrange the ideal gas equation (pV = nRT) to solve for V:
V = nRT / p = (3.66 × 10⁻³ × 8.31 × 298) / 101,000 = 8.98 × 10⁻³ m³ (or 8.98 dm³ / 8980 cm³). - Check units and significant figures:
Expressed to 3 significant figures with correct units (e.g., m³ , dm³ , or cm³ ).
✅ Part (ii): Percentage Uncertainty (1 Mark)
Uncertainty = (±0.5 / measured volume) × 100
Using 89.9 cm³ (or candidate's volume converted to cm³): (0.5 / 89.9) × 100 = 0.556% (Accept 0.56% or 0.6%).
✅ Part (iii): Percentage Purity (3 Marks)
Method summary:
- Calculate expected moles of CuO from pure malachite: 2 × 3.66 × 10⁻³ = 7.33 × 10⁻³ mol.
- Calculate expected mass of CuO: 7.33 × 10⁻³ × 79.5 = 0.582 g.
- Use purity formula: (Actual mass / Expected mass) × 100 = (0.571 / 0.582) × 100 = 98.0% .
Topics
Physical Chemistry · Inorganic Chemistry · Core Practicals · Topic 1: Atomic Structure and the Periodic Table · Topic 5: Formulae, Equations and Amounts of Substance · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 1: Measuring the molar volume of a gas · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 15: Transition Metals
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.