Edexcel A-Level Chemistry AS Paper 1, June 2023: Question 7

6 marks · Medium difficulty · Calculations

Draw a dot-and-cross diagram for potassium chloride, deduce and justify the order of increasing size for various ions, and calculate the concentration of calcium ions in a saturated calcium hydroxide solution.

Practise this question

Question

Question text

7 This question is about ions.

(a) Draw a dot‑and‑cross diagram of the ionic compound potassium chloride.

Show outer shell electrons only.

(1)

(b) Deduce the order of increasing size of the ions:

Br–, Ca2+, Cl–, K+

Justify your answer.

(4)

of solution.

Under these conditions the calcium hydroxide is fully dissociated into ions.

What is the concentration of the calcium ions in this solution, in mol dm–3 ?

(1)

A 0.0233

B 0.0303

C 0.0431

D 0.0597

(Total for Question 7 = 6 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

7(a) Example of diagram (1)

• diagrams for K+ ions

and

diagram for Cl‒ ion

Allow positive and negative charges anywhere on the ions, with or

without brackets

Allow any symbol for electrons

Allow no electrons around the potassium

Ignore inner shells of electrons

Number

7(b) A description that makes reference to the following Marks can be scored by comparison of pairs of ions, so for (4)

points: example a comparison of Ca2+ and K+ saying they have the same

number of electrons and a similar comparison of Ca2+ and Cl‒

scores M1

• Ca2+, Cl‒ and K+ have the same number of Ignore they have the same number of shells of electrons

electrons / are isoelectronic (1)

• Br‒ has one more shell of electrons than the Allow Br‒ has more shells of electrons

other ions so it is largest (1)

‒ + 2+ Allow from Cl‒ to K+ to there are two more protons in the nucleus

• from Cl to K to Ca there are more protons

(in the nucleus) (1) and from K+ to Ca2+ there is one more proton in the nucleus.

Ignore Ca2+ has the highest charge, then K+ then Cl‒ without

explanation

Ignore nuclear charge unless clearly explained as the number of

protons rather than charge on the ion

Allow Ca2+, K+, Cl‒, Br‒

• so (the order of increasing size of ion is)

Ca2+ < K+ < Cl‒ < Br‒ (1) Allow order of decreasing size if described as such.

This mark is stand alone

Question

Answer Mark

Number

7(c) The only correct answer is A (0.0233) (1)

B is not correct because the value assumes the formula of calcium hydroxide is CaOH

C is not correct because the value assumes that 17.3g is the mass of calcium not calcium hydroxide

D is not correct because this assumes the relative formula mass of calcium hydroxide is 29 (the sum of the atomic numbers)

(Total for Question 7 = 6 marks)

How to answer it

Question 7: Ions, Ionic Radii, and Solution Calculations

What this question tests

This question assesses core physical and inorganic chemistry concepts: drawing ionic dot-and-cross diagrams for ionic lattices, explaining trends in ionic radii using shielding and nuclear charge (isoelectronic ions), and performing quantitative solution chemistry calculations involving concentration, molar mass, and stoichiometry.

Part (a) • 1 Mark

Dot-and-Cross Diagram for Potassium Chloride

✅ Correct Answer

Draw separate representations for the K⁺ ion and the Cl⁻ ion:

K⁺ ion: Shown with an empty outer shell (or showing 8 electrons if preferred) with a + charge, often enclosed in square brackets.
Cl⁻ ion: Shown with 8 outer shell electrons (using a mix of dots and crosses) with a - charge, enclosed in square brackets.

💡 Key Knowledge

Ionic bonding involves the complete transfer of electrons from metal to non-metal.
Potassium loses its single 4s electron to form K⁺ (retaining a full 3s² 3p⁶ outer shell).
Chlorine gains one electron to complete its 3p subshell, forming Cl⁻ .

❌ Common Errors

Students often lose marks by forgetting to include the overall charges ( + and - ) on the ions, or by failing to use square brackets around the ions.

Mark scheme note: Square brackets and charges can be placed anywhere on or near the ions. Inner shells of electrons can be ignored.

Part (b) • 4 Marks

Order of Increasing Ionic Size and Justification

✅ Correct Answer

Order of increasing size: Ca²⁺ < K⁺ < Cl⁻ < Br⁻

🧠 Exam Technique & Mark Breakdown

Mark 1: Identify that Ca²⁺ , K⁺ , and Cl⁻ are isoelectronic (have the same number of electrons).
Mark 2: State that Br⁻ has an additional electron shell, making it the largest.
Mark 3: Explain the trend across the isoelectronic ions: as atomic number increases ( Ca²⁺ to K⁺ to Cl⁻ ), the nuclear charge increases, pulling the same number of electrons closer.
Mark 4: State the correct full sequence clearly.

❌ Common Errors

Referring simply to "nuclear charge" without linking it to the number of protons or failing to explicitly mention that Br⁻ has an extra electron shell will cause candidates to miss crucial explanatory marks.

Part (c) • 1 Mark

Concentration of Calcium Ions Calculation

✅ Correct Answer

A ( 0.0233 mol dm⁻³ )

📐 Step-by-Step Calculation

Formula of calcium hydroxide: Ca(OH)₂
Molar mass of Ca(OH)₂: 40.1 + (2 × 16.0) + (2 × 1.0) = 74.1 g mol⁻¹
Moles of Ca(OH)₂: 17.3 g / 74.1 g mol⁻¹ = 0.23347 mol
Concentration of Ca(OH)₂: 0.23347 mol / 10.0 dm³ = 0.023347 mol dm⁻³
Stoichiometry check: Ca(OH)₂ → Ca²⁺ + 2OH⁻ . The mole ratio of Ca(OH)₂ to Ca²⁺ is 1:1, so [Ca²⁺] = 0.0233 mol dm⁻³ .

❌ Distractor Analysis

B (0.0303): Assumes incorrect formula CaOH .
C (0.0431): Uses mass of calcium alone instead of calcium hydroxide.
D (0.0597): Uses incorrect molar mass (sum of atomic numbers instead of relative atomic masses).

Topics

Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · 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, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.