Edexcel A-Level Chemistry Paper 3, June 2022: Question 3

13 marks · Medium difficulty · Practical Techniques and Data Analysis

Deduce the formulae of two salts X and Y using qualitative tests, and determine the number of moles of water of crystallisation in Y using stoichiometry and experimental analysis.

Practise this question

Question

A three-part question about two salts, X and Y. Part (a) asks to describe a flame test and identify the metal ion in X given a yellow flame. Part (b) describes testing salt X with hydrochloric acid and barium chloride, resulting in a white precipitate, to deduce the formula of X. Part (c) investigates the water of crystallisation in hydrated potassium carbonate Y, including an experimental diagram of a crucible on a pipeclay triangle being heated, errors relating to the lid and heating time, and a calculation of the value of n from percentage composition.
Question text

3 A group of students design and carry out experiments to deduce the formulae of

two salts, X and Y.

X contains one cation and one anion.

Y contains water of crystallisation.

(a) (i) A flame test is carried out on X.

Describe how to carry out a flame test.

(3)

(ii) The colour of the flame is yellow.

Give the formula of the metal ion present in salt X.

(1)

(b) A sample of X is placed in a test tube and dissolved in deionised water.

The solution is acidified with hydrochloric acid and barium chloride solution

is added.

A white precipitate forms.

(i) Give the formula of the anion present in X.

(1)

(ii) Deduce the formula of X, using your answers to (a)(ii) and (b)(i).

(1)

(c) Y is identified as hydrated potassium carbonate, K2CO3·nH2O.

Two of the students were asked to determine the number of moles of

water of crystallisation, n, in Y using the procedure shown:

6 • weigh a sample of hydrated Y into a pre-weighed crucible

• place a lid loosely on the crucible and heat it for five minutes to remove the*P67095A0636*

water of crystallisation

• allow the crucible and lid to cool, remove the lid and then reweigh the

crucible with its contents.

lid

hydrated Y

crucible

pipeclay triangle

heat

(i) The first student carried out the experiment but forgot to use the lid.

Explain how this mistake would affect the calculated value of n.

(2)

(ii) The second student carried out the experiment but heated the apparatus for

only one minute.

Explain how this mistake would affect the calculated value of n.

(2)

*P67095A0736*

(iii) In an accurate experiment, Y is found to consist of 71.9% K2CO3 by mass.

Calculate the value of n.

(3)

(Total for Question 3 = 13 marks)

Mark scheme

Show the mark scheme The mark scheme provides detailed points for each part of question 3. For 3(a)(i), marks are awarded for mentioning nichrome/platinum wire, dipping in HCl, and placing in a non-luminous flame. Subsequent parts award marks for specific ion formulae such as Na+ and SO42-, explanations of errors during the crucible heating experiment involving loss of solid or incomplete dehydration, and calculation steps to find n = 3 for the water of crystallisation.

Question

Answer Additional Guidance Mark

Number

3(a)(i) A description that makes reference to the following points: (3)

• any mention of platinum / nichrome wire / loop (1) Allow NiCr for nichrome

Allow silica rod

Ignore ‘inoculating’ / ‘sterilising’

Do not award just nickel or chromium

• dip the wire into (clean / fresh concentrated) hydrochloric Allow mention of HCl before or after dipping wire

acid / HCl (1) into solid e.g. cleaning or mixing solid and HCl to

make a paste

Ignore concentration of HCl

Ignore just ‘acid’ / other acids specified

Do not award HCl reacting with flame test wire

• dip the (wet) wire into the solid and place in a (non-luminous Allow salt / compound / paste / sample / solution

/ roaring / blue Bunsen) flame (1) for solid

Allow through the flame / on the edge of the flame

for in the flame

Do not award element / metal for solid

Do not award over / above / under the flame

Do not award just ‘into a Bunsen’

Do not award ‘burn in flame’

Do not award flame if Bunsen has air-hole closed /

safety flame

Question

Answer Additional guidance Mark

Number

3(a)(ii) • Na+ Ignore state symbols (1)

Ignore sodium / sodium ion

Do not award incorrect charge

Question

Answer Additional Guidance Mark

Number

3(b)(i) Ignore state symbols (1)

• SO 2– Ignore sulfate(VI) / sulfate/ sulphate

Do not award sulfate(IV) / sulfite / hydrogensulfate

Do not award incorrect charge

Question

Answer Additional Guidance Mark

Number

3(b)(ii) Ignore state symbols (1)

• Na2SO4 Ignore names

Allow TE from other ions, with correct charges, given in

(a)(ii) and (b)(i)

Allow large numbers e.g. Na2SO4 but not superscripts e.g.

Na2SO4

Question

Answer Additional Guidance Mark

Number

3(c)(i) An explanation that makes reference to the following points: (2)

Allow solid / product / crystals for ‘salt’

• (not using a lid means) some of salt Y could be lost from Allow ‘salt spits / jumps out’ / ‘salt escapes’ from

crucible during heating (1) crucible

Ignore gas escapes

Do not award ‘salt evaporates’

• (mass loss greater than expected), so n / amount of water (of M2 dependent on M1 or salt evaporates

crystallisation) greater (than expected) (1)

Question

Answer Additional Guidance Mark

Number

3(c)(ii) An explanation that makes reference to the following points: (2)

• (heating for only 1 minute may mean) not all the water (of Allow evaporated / boiled off for removed

crystallisation) has been removed (1) Allow (only) partial dehydration

Ignore incomplete reaction

• (mass loss less than expected), so n / amount of water (of M2 dependent on M1 or incomplete reaction

crystallisation) less (than expected) (1)

Question

Answer Additional Guidance Mark

Number

3(c)(iii) Example of calculation (3)

K2CO3 H2O

Moles = 71.9 / (138.2) (100 - 71.9) / 18

• calculation of moles of K2CO3 (1) = 0.52026 = 1.56111

Ratio = = 0.52026 / = 1.56111 /

• calculation of moles of H2O (1) 0.52026 0.52026

= 1 = 3

n = 3

• deduction of n (1) Accept use of 0.719 / 0.281 in M1

Allow TE from M1

Allow use of 138 for Mr of K2CO3 – gives 0.52101

Ignore SF including 1SF in M1 and M2

M3 must be 1 SF

Accept alternative methods e.g.

138.2 = 0.719 (1)

138.2 + 18n

38.8342 = 12.942n (1) so n = 3 (1)

or

Mr of hydrated salt = 138.2 = 192.2 (1)

0.719

mass of water = 192.2 – 138.2 = 54 (1)

n = 54/18 = 3 (1)

or

138.2 = 71.9% so 28.1% is water (1) 138.2 x 28.1 = 54 (1)

71.9

n = 54/18 = 3 (1)

Correct answer with no working scores (1)

Correct answer with some correct working scores (3)

(Total for Question 3 = 13 marks)

How to answer it

Analysis of Salts X and Y: Identification and Water of Crystallisation

📌 What this question tests

This question assesses core practical skills and inorganic ion tests. You are tested on your ability to describe standard laboratory procedures (flame tests), recall ion-specific chemical tests (sulfate identification using acidified barium chloride), formulate ionic formulas, evaluate practical errors in thermal decomposition experiments, and execute mole calculations involving percentage mass and water of crystallisation.

Question 3(a) — Flame Testing Salt X

Part (i) & (ii): Procedure and Cation Identification

✅ Correct Answer: Part (i)

  • Mention use of platinum or nichrome wire/loop (or silica rod).
  • Dip the wire into concentrated hydrochloric acid (HCl).
  • Dip the wet wire into the solid and place it into a non-luminous (roaring/blue) Bunsen flame.

✅ Correct Answer: Part (ii)

Na⁺

Award 1 mark. Ignore state symbols or writing "sodium ion" in words. Do not award incorrect charges.

💡 Key Knowledge

A yellow flame is characteristic of sodium ions ( Na⁺ ). Concentrated HCl is used to convert metal compounds into volatile metal chlorides, which vaporise readily in the flame.

❌ Common Errors

  • Forgetting to dip the wire in acid first.
  • Placing the wire in a luminous (yellow safety) Bunsen flame instead of a roaring blue flame.
  • Writing formulas with incorrect charges like Na²⁺ or including unnecessary state symbols.

Question 3(b) — Anion Identification and Formula of X

Part (i) & (ii): Testing for Anions and Deducing Formula

✅ Correct Answer: Part (i)

SO₄²⁻

Award 1 mark for sulfate ion. Accept "sulfate(VI)". Reject sulfite ( SO₃²⁻ ) or hydrogensulfate ( HSO₄⁻ ).

✅ Question 3(b)(ii) Formula of X

Na₂SO₄

Award 1 mark. Allows consequential marking (TE) if ions from (a)(ii) and (b)(i) were combined correctly with balanced charges.

🧠 Exam Technique

When testing for sulfate ions, acidifying with hydrochloric acid is essential to remove carbonate or sulfite impurities that would also form a white precipitate with barium chloride.

Question 3(c) — Water of Crystallisation & Practical Analysis

Parts (i) & (ii): Evaluating Practical Errors

✅ Correct Answer: Part (i) (No Lid)

  • Some solid Y is lost/escapes (splits out) from the crucible during heating. (1 mark)
  • Calculated mass loss is too large, leading to a calculated n (amount of water) that is greater than expected. (1 mark - dependent on M1)

✅ Correct Answer: Part (ii) (Insufficient Heating)

  • Not all water of crystallisation has been removed (incomplete dehydration). (1 mark)
  • Calculated mass loss is too small, leading to a calculated n that is smaller than expected. (1 mark - dependent on M1)

Question 3(c)(iii) — Calculation of n

Step-by-Step Calculation Guide

📐 Calculation Breakdown for K₂CO₃ · nH₂O

Step 1: Find percentage/mass of water

If K₂CO₃ is 71.9% , then water ( H₂O ) is 100 - 71.9 = 28.1% .

Step 2: Calculate moles of each component (per 100g sample)

  • Molar mass of K₂CO₃ = (2 × 39.1) + 12.0 + (3 × 16.0) = 138.2 g mol⁻¹
  • Moles of K₂CO₃ = 71.9 / 138.2 = 0.520 mol
  • Moles of H₂O = 28.1 / 18.0 = 1.561 mol

Step 3: Determine simplest whole-number ratio ( n )

  • Ratio ( K₂CO₃ : H₂O ) = 0.520 / 0.520 : 1.561 / 0.520
  • Ratio = 1 : 3.00
  • Therefore, n = 3
Award 3 marks total: 1 mark for moles of K₂CO₃, 1 mark for moles of H₂O, 1 mark for deduction of n = 3 (must be an integer).

❌ Common Calculation Traps

  • Using atomic mass 138 instead of 138.2 for K₂CO₃ (though mark scheme accepts 138, precision is encouraged).
  • Forgetting to divide by the smallest number of moles to find the ratio.
  • Leaving n as a decimal instead of rounding to the nearest whole number.

Topics

Inorganic Chemistry · Physical Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 5: Formulae, Equations and Amounts of Substance · Core Practical 7: Identify unknown organic liquids and inorganic solids

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.