Edexcel A-Level Chemistry Paper 3, June 2024: Question 3
13 marks · Medium difficulty · Calculations
Calculate the relative formula mass of M2SO4 and identify M from a gravimetric precipitation experiment involving barium chloride, and explain flame test principles and experimental errors.
Practise this questionQuestion
Question text
3 A student carried out an experiment to identify the Group 1 metal ion, M+, in a
sulfate, M2SO4.
Procedure
Step 1 A 0.48 g sample of M SO was added to about 50 cm3 of deionised water
in a beaker. The mixture was stirred with a glass rod until the sample
dissolved completely.
Step 2 An excess of acidified barium chloride solution, BaCl2(aq), was added very
slowly to the solution from Step 1 to ensure the crystals of barium sulfate that
formed were as large as possible.
Step 3 The barium sulfate crystals were collected by gravity filtration onto a dry filter
paper of known mass.
Step 4 The crystals were rinsed using deionised water.
Step 5 The filter paper containing the crystals was dried in a warm oven for an hour
and then weighed.
Results
Mass of dry filter paper = 0.17g
Mass of dry filter paper and barium sulfate crystals = 1.19g
(a) Give a reason, with reference to the filtration in Step 3, why the formation
of extremely small crystals in Step 2 would lead to a lower mass of dry
barium sulfate in Step 5.
(1)
(b) Explain why the crystals are rinsed in Step 4.
(2)
(c) Calculate the relative formula mass of M2SO4 and hence identify M.
M2SO*P74455A0532*4(aq)+BaCl2(aq)→BaSO4(s)+2MCl(aq)
[Mr of BaSO4 = 233.4]
(3)
(d) Another student carrying out the experiment placed the glass rod on the
laboratory bench when the stirring was complete in Step 1.
(i) Explain what effect this would have on the calculated value of the relative
formula mass of M2SO4.
(2)
(ii) Describe how the student should amend the procedure.
(1)
(e) Another way to identify a metal ion is a flame test.
(i) Explain what causes the flame colour in such a test.
(2)
… *P74455A0632*
(ii) Give one advantage and one disadvantage of using a flame test to identify a
metal cation, rather than the method followed by the students in Steps 1–5.
(2)
(Total for Question 3 = 13 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
3(a) (1)
• (crystals will be small enough) to get through (pores in) the filter paper Ignore references to just not being filtered
Do not award references to crystals being
‘stuck’ on the filter paper or dissolving
Question
Answer Additional Guidance Mark
Number
3(b) An explanation that makes reference to the following points: (2)
• to remove (excess) BaCl2(aq) / MCl(aq) (1) Allow to remove remaining solution
Allow reference to removing filtrate
Ignore reference to residue
• which would otherwise crystallise / add to mass (of sulfate) (1) Allow reference to ‘change the mass’
Do not award a decrease in mass
If no other mark awarded then allow (1) for
to remove (soluble) impurities
Do not award removal of solid impurities
Question
Answer Additional Guidance Mark
Number
3(c) (3)
• calculation of moles of BaSO (1) 1.02 ÷ 233.4 = 4.3702 × 10–3 / 0.0043702 (mol)
(1) 0.48 ÷ 4.3702 × 10–3 = 109.84 / 110 (g mol−1)
• calculation of RFM of M2SO4
Ignore SF except 1 SF for M1 and M2
• deduction of identity of M (1) (109.84 – (32.1 + 64) = 13.74 hence Ar of M = 6.87 / 6.9 ) so M = Li
Allow Li+
Ignore SF for M3
Allow TE from M1 to M2
Allow TE from M2 to M3 provided value is positive and linked to
an appropriate metal that can form a +1 ion
Correct answer without working scores (1)
Question
Answer Additional Guidance Mark
Number
3(d)(i) An explanation that makes reference to the following points: (2)
• (some of) M2SO4 (solution) would be lost Allow just some of the solution would be lost
(from the beaker) (1)
• so (calculated) moles/amount (of M2SO4) would be Allow reference to moles of BaSO4 for M2SO4
less Ignore less mass of BaSO4
and
(calculated) formula mass would be more/higher (1) Allow M2 for answers that show understanding of the
negligible change (of RFM)
M2 dependent upon M1 attempt
Allow M2 for a TE such as the moles of M2SO4 would be
more which results in a lower RFM provided the answer
states that the mass has increased
Question
Answer Additional Guidance Mark
Number
3(d)(ii) (1)
• rinse glass rod with deionised/distilled water Allow wash for rinse
into the beaker Allow ‘to the solution’ for “into the beaker”
Do not award rinsings go down the sink
References to the use of a magnetic stirrer can score if
rinsing is mentioned of the ‘flea’
Question
Answer Additional Guidance Mark
Number
3(e)(i) An explanation that makes reference to the following points: Electrons need to be mentioned once in M1 or M2 (2)
• heat causes electrons to be promoted / excited / move Allow just ‘energy (from flame) causes electrons to be
to a higher energy level (1) promoted/excited’
• light is emitted when electrons fall back Accept reference to photons for light
(to lower level / ground state) (1) Allow released/ given out for emitted
Ignore reference to just energy
Do not award reference to complementary
colours/absorption colour
Do not award reference to reflected/transmitted
Question
Answer Additional Guidance Mark
Number
3(e)(ii) An answer that makes reference to the following points: (2)
• Advantage – simpler (process) / quicker (process) (1) Allow just ‘easier’
Ignore just ‘cheaper’
Ignore references to more accurate
• Disadvantage – flame colours are subjective (by eye) Allow several metal (ions) produce red flames / not all
/ some metal (ions) produce very similar flame metal (ions) produce a flame colour (in the visible
colours (by eye) / often more than one colour may be range)
seen (due to contamination) (1) If metals are named then their colours must be correct
(Total for Question 3 = 13 marks)
How to answer it
Identifying a Group 1 Metal Ion in a Sulfate (M₂SO₄)
What this question tests
This practical-based question assesses your understanding of gravimetric analysis, stoichiometry, titration/precipitation techniques, experimental errors, and atomic emission (flame tests). You will need to calculate moles, determine relative formula masses (Mr), account for practical losses, and evaluate the analytical advantages and limitations of chemical tests vs. physical flame tests.
Effect of Crystal Size on Filtration
✅ Correct Answer
The extremely small crystals would be small enough to pass straight through the pores of the filter paper.
❌ Common Errors
Students often incorrectly claim that the crystals would get "stuck" or "dissolve". Remember: smaller particles have a higher surface-to-volume ratio, and if particle dimensions are smaller than filter pores, separation fails entirely.
Rinsing the Precipitate
✅ Correct Answer
To remove excess barium chloride (BaCl₂) or MCl solution, which would otherwise crystallize upon drying and artificially increase the measured mass of the barium sulfate precipitate.
💡 Key Knowledge
Precipitation gravimetry requires high purity. Any unreacted reagents dissolved in the residual moisture will leave behind solid residues when heated in the oven, leading to a false high mass reading.
Gravimetric Calculation & Identification of M
M₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2MCl(aq)
Given data: Mass of M₂SO₄ = 0.48 g | Mass of dry filter paper = 0.17 g | Mass of filter paper + BaSO₄ = 1.19 g | Mᵣ of BaSO₄ = 233.4
📐 Step-by-Step Calculation
- Step 1: Find mass of BaSO₄ precipitate
1.19 g − 0.17 g = 1.02 g - Step 2: Calculate moles of BaSO₄
Moles = 1.02 ÷ 233.4 = 4.3702 × 10⁻³ mol - Step 3: Use stoichiometry to find moles of M₂SO₄
From the equation, 1 mole of M₂SO₄ produces 1 mole of BaSO₄.
Therefore, moles of M₂SO₄ = 4.3702 × 10⁻³ mol - Step 4: Calculate Mᵣ of M₂SO₄
Mᵣ = Mass ÷ Moles = 0.48 ÷ (4.3702 × 10⁻³) = 109.84 (or 110 g mol⁻¹) - Step 5: Identify metal M
Let atomic mass of M be $A_r$.
2(Aᵣ) + 32.1 + (4 × 16.0) = 109.84
2(Aᵣ) + 96.1 = 109.84 ⇒ 2(Aᵣ) = 13.74 ⇒ Aᵣ = 6.87.
Comparing with the periodic table, Aᵣ of Lithium (Li) = 6.9.
Identity: Li⁺ (Lithium)
Practical Error Analysis & Procedural Amendment
❌ (i) Effect of leaving the glass rod on the bench
Some of the M₂SO₄ solution would be lost on the glass rod. This means fewer moles of M₂SO₄ react, yielding less BaSO₄ precipitate. Consequently, the calculated moles of M₂SO₄ will appear lower, leading to an overestimated/higher relative formula mass (Mᵣ).
💡 (ii) Required Procedure Amendment
The student must rinse the glass rod with deionised water directly into the beaker before removing it, ensuring all dissolved substance enters the reaction mixture.
Flame Tests: Theory, Advantages, and Disadvantages
💡 (i) Mechanism of Flame Colour
- Heat energy from the flame is absorbed, causing electrons to be promoted/excited to higher energy levels.
- When these excited electrons fall back down to lower energy levels (or ground state), energy is released as light/photons of a specific wavelength.
⚖️ (ii) Advantages & Disadvantages vs Gravimetry
- Advantage: Much simpler and quicker to carry out than multi-step filtration and drying.
- Disadvantage: Flame colours are subjective (observed by eye), many metal ions produce very similar or overlapping colours, and contamination can cause multiple conflicting colours to appear.
Topics
Inorganic Chemistry · Physical Chemistry · Core Practicals · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 5: Formulae, Equations and Amounts of Substance · Topic 1: Atomic Structure and the Periodic Table
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.