Edexcel A-Level Chemistry AS Paper 1, June 2023: Question 2
10 marks · Medium difficulty · Calculations
Calculate relative formula mass, amount in moles, describe flame and precipitation tests for cations and anions in the double salt K2Mg(SO4)2·6H2O.
Practise this questionQuestion
Question text
2 This question is about the double salt K2Mg(SO4)2.6H2O.
A double salt is a salt containing more than one cation or more than one anion.
(a) (i) Give the reason why the term relative formula mass is used, rather than
relative molecular mass, when discussing K2Mg(SO4)2.6H2O.
(1)
(ii) Give the reason why relative formula mass has no mathematical units.
(1)
(iii) What is the relative formula mass of K2Mg(SO4)2.6H2O?
Use the Periodic Table as a source of data.
(1)
A 206.0
B 322.7
C 402.7
D 595.3
(iv) Calculate the amount of substance, in mol, in 25.0g of K2Mg(SO4)2.6H2O.
(1)
(b) Cations may be identified using a flame test on a solid or by the addition of
aqueous sodium hydroxide to an aqueous solution containing the cation.
Describe the expected results of performing these tests on K2Mg(SO4)2.6H2O.
Include the observation for each cation in each test.
(4)
… 4
… *P71926A0428*
(c) The test used to show the presence of the anion in K2Mg(SO4)2.6H2O gives a
white precipitate as the positive result.
(i) What are the reagents used for this test?
(1)
A dilute hydrochloric acid and aqueous silver nitrate
B dilute sulfuric acid and aqueous silver nitrate
C dilute hydrochloric acid and aqueous barium chloride
D dilute sulfuric acid and aqueous barium chloride
(ii) Write an ionic equation, including state symbols, for the formation of the
white precipitate in the test for the anion in the double salt.
(1)
(Total for Question 2 = 10 marks)
Mark scheme
Show the mark scheme
Question Answer Additional Guidance Mark
Number
2(a)(i) An answer that makes reference to the following (1)
point:
• (relative formula mass is used because) the Allow it is not molecular
salt is a giant (ionic) structure / it is not a Allow it is not a molecule
molecular structure Allow it is not made up of molecules
Allow molecular mass is only used for molecules
Ignore it is a compound
Ignore just ionic
Do not award it is a molecule / molecules
Number
2(a)(ii) An answer that makes reference to the following (1)
point:
• it is measured relative to / compared to 1/12 of Allow it is the mass of an atom relative to 1/12 of the mass of an
the mass of an atom of carbon-12 atom of carbon-12
Allow it has the units of grams ÷ grams / g ÷ g (so no units)
Allow relative measurements are a comparison so have no units
Allow because it is not an actual mass but a relative mass
Ignore just ‘it is a relative mass’
Question
Answer Mark
Number
2(a)(iii) The only correct answer is C (402.7) (1)
A is not correct because this is the sum of the atomic numbers not masses
B is not correct because this has 6H2 but only one O from the water of crystallisation
D is not correct because this is the mass if the sulfate were (S4O4)2
Number
2(a)(iv) An answer that makes reference to the following Example of calculation: (1)
point:
• calculation of amount of substance 25.0 = 0.062081 / 0.0621 / 0.062 / 6.2081 x 10-2 / 6.21 x 10-2
402.7 / 6.2 x 10-2 (mol)
Allow answer in mmol
Allow TE on incorrect multiple-choice answer(A gives 0.12136, B
gives 0.077471, D gives 0.041996)
Ignore SF except 1 SF
Number
2(b) An answer that makes reference to the following Penalise omission of ions once only (4)
points:
• lilac flame due to potassium ions / K+ (1) Allow potassium atoms / potassium / K would produce a lilac
flame
• magnesium ion / Mg2+ gives no flame colour Do not award magnesium atoms / magnesium / Mg / gives no
(1) flame colour
• white precipitate (of Mg(OH)2 / from the
magnesium / from Mg2+) (1) Allow any link to magnesium
If formula of magnesium hydroxide is given it must not be
incorrect e.g MgOH or MgOH2
• potassium ions do not form a precipitate (with
sodium hydroxide) (1) Allow KOH is soluble
Question
Answer Mark
Number
2(c)(i) The only correct answer is C (dilute hydrochloric acid and barium chloride solution) (1)
A is not correct because this would give a white precipitate of silver chloride due to a reaction hydrochloric acid and silver nitrate
B is not correct because sulfate ions are also present in the sulfuric acid and silver nitrate is used to test for halides
D is not correct because this would give a white precipitate of barium sulfate due to the sulfuric acid
Number
2(c)(ii) An answer that makes reference to the following (1)
point:
For answers C or D
• balanced ionic equation including state Ba2+(aq) + SO2‒ (aq) → BaSO (s)
symbols
For answers A or B
Allow TE for 2Ag+(aq) + SO2‒ (aq) → Ag SO (s)
42 4
Allow multiples
Do not award uncancelled spectator ions
Do not award Ag+(aq) + Cl‒(aq) → AgCl(s)
(Total for Question 2 = 10 marks)
How to answer it
Analysis of the Double Salt K₂Mg(SO₄)₂·6H₂O
What this question tests
This question assesses your understanding of fundamental chemical terminology (relative formula mass vs. relative molecular mass), mole calculations involving hydrated double salts, qualitative analytical tests for specific cations (flame tests and aqueous sodium hydroxide reactions), and testing for sulfate anions using acidified barium chloride with ionic equations.
Question 2(a) — Terminology and Molar Mass Calculations
(i) Correct Answer
Relative formula mass is used because the salt has a giant ionic structure (or is not made of discrete molecules).
💡 Key Knowledge
- Relative Molecular Mass (Mr): Strictly used only for substances that exist as discrete molecules (covalent structures).
- Relative Formula Mass: Used for ionic compounds or giant structures that do not form simple molecules.
(ii) Correct Answer
It has no units because it is a ratio measured relative to 1/12 of the mass of an atom of carbon-12.
(iii) Correct Answer: C (402.7)
M(K₂Mg(SO₄)₂·6H₂O) = (2 × 39.1) + 24.3 + 2 × (32.1 + (4 × 16.0)) + 6 × ((2 × 1.0) + 16.0) = 402.7
❌ Common Errors in (iii)
Students often select option B by forgetting to multiply all atoms in the sulfate group by 2, or make arithmetic slips with water of crystallisation.
📐 (iv) Step-by-Step Calculation: Amount of Substance
- Identify formula and Mr: Molar mass of K₂Mg(SO₄)₂·6H₂O = 402.7 g mol⁻¹
- Recall formula: Amount (mol) = Mass ÷ Molar Mass
- Substitute values: 25.0 g ÷ 402.7 g mol⁻¹ = 0.062081 mol
- Apply appropriate significant figures: 0.0621 mol (or 6.21 × 10⁻² mol ).
Question 2(b) — Qualitative Analysis of Cations
✅ Expected Observations (4 Marks)
- Flame test on solid: Lilac flame observed (due to potassium ions, K⁺).
- Flame test on Mg: Magnesium ions (Mg²⁺) produce no flame colour.
- Addition of NaOH(aq): Forms a white precipitate of magnesium hydroxide, Mg(OH)₂.
- Effect of excess NaOH: Potassium ions (K⁺) do not form a precipitate (soluble/no change).
🧠 Exam Technique & Common Pitfalls
Crucial Distinction: You must refer to the ions (K⁺ and Mg²⁺) rather than unreacted atoms. Saying "magnesium gives no flame colour" is accepted, but stating "magnesium atoms give..." can lose marks depending on strictness. Ensure formulas for precipitates like Mg(OH)₂ are written correctly if named.
Question 2(c) — Testing for Sulfate Anions
(i) Correct Answer: C
Dilute hydrochloric acid and aqueous barium chloride.
❌ Why other options are incorrect
Using sulfuric acid (B or D) introduces sulfate ions into the mixture, resulting in a false positive. Silver nitrate (A or B) is used for testing halides, not sulfates.
(ii) Correct Ionic Equation
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
💡 Key Knowledge
State symbols are mandatory for ionic precipitation equations. The white precipitate formed is barium sulfate ( BaSO₄ ).
Topics
Physical Chemistry · Inorganic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.