AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 4

Mark scheme

Show the mark scheme

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 a mixture designed as a useful 1 AO1

product 5.8.1.2

04.2 an answer of 1.8 × 102 (mol) AO2

gains 4 marks 5.8.1.2

5.3.2.1

mass = 14 520 g 1

14 520 allow correct substitution of 1

(=)

80 (mol) incorrectly converted mass

must use Mr given (80) to gain

marks in steps 2 and 3

(=) 181.5 (mol) 1

(=) 1.8 × 102 (mol) allow answer correctly given in 1

standard form to correct sig figs

from an incorrect calculation

04.3 max 2 marks for incorrect AO1

reference to bonding or 5.2.2.3

structure or particles

(giant) lattice allow giant structure 1

ionic 1

strong bonds or strong do not accept strong 1

electrostatic forces intermolecular forces / bonds

large amounts of energy needed ignore heat 1

to overcome

Total 9

How to answer it

Fertilisers as Formulations

What this question tests

You need to know what a formulation is, use the moles = mass ÷ Mr calculation with unit conversion, and explain why an ionic compound has a high melting point using structure and bonding ideas.

Question 04.1

What is a formulation?

✅ Correct answer

A mixture designed as a useful product.

This is the exact mark scheme idea. For 1 mark, you only need a clear definition.

💡 Key knowledge

A formulation is not just any mixture.
It is a mixture made to have a specific purpose or useful property.
Examples: fertilisers, paints, medicines, fuels, cleaning products.

🧠 Exam technique

Use the word mixture.
Include the idea that it is designed or useful.
One short sentence is enough for full marks.

❌ Common errors

Saying “a substance” is too vague.
Saying “a pure substance” is wrong.
Missing the idea of being made for a purpose can lose the mark.

Question 04.2

Calculate the number of moles of ammonium nitrate

Mass = 14.52 kg, Mr(NH₄NO₃) = 80, give answer in standard form to 2 s.f.

📐 Calculations: step-by-step

Convert the mass to grams:
14.52 kg = 14 520 g
Use the formula:
moles = mass ÷ Mr
Substitute the values:
moles = 14 520 ÷ 80
Calculate:
moles = 181.5 mol
Write in standard form to 2 s.f.:
1.8 × 10² mol

✅ Full-mark answer

1.8 × 10² mol

Marks are awarded for:
• converting kg to g
• using the correct formula
• getting the correct calculated value
• giving the answer in standard form to 2 significant figures

🧠 Exam technique

Always check the unit of mass: the formula uses g, not kg.
Show your working clearly so you can gain method marks.
Final answer must match the instruction: standard form and 2 significant figures.
If your working is slightly wrong but the final answer is correctly rounded from that working, you may still gain some marks.

❌ Common errors

Using 14.52 instead of converting to 14 520 g.
Dividing by the wrong Mr.
Writing the answer as 181.5 mol instead of standard form.
Giving the answer to the wrong number of significant figures.
Forgetting units: the final answer should include mol.

💡 Key knowledge

moles = mass ÷ Mr
1 kg = 1000 g
Standard form for this answer is 1.8 × 10²

Question 04.3

Explain why potassium chloride has a high melting point

✅ Correct answer

Potassium chloride has a giant ionic lattice.
It contains oppositely charged ions held together by strong electrostatic forces of attraction.
A large amount of energy is needed to overcome these forces.
Therefore potassium chloride has a high melting point.

💡 Key knowledge

Potassium chloride is an ionic compound.
Ionic compounds form a giant lattice of ions.
The melting point is high because many strong attractions must be broken, not just one bond.

🧠 Exam technique

To score full marks, link structure + bonding + energy.
A top answer usually includes the phrases giant lattice, ionic, and strong electrostatic forces.
Say that large amounts of energy are needed to overcome the attractions.

❌ Common errors

Talking about intermolecular forces is wrong here.
Calling it a “simple molecule” is wrong.
Saying “strong bonds” without explaining the ionic lattice can be too vague.
Just saying “it takes a lot of heat” is not enough unless you explain why.

📐 What you would draw if asked for a diagram

If a diagram were required, draw a regular repeating lattice of alternating K⁺ and Cl⁻ ions. Show the ions arranged in a fixed pattern and label the electrostatic attraction between opposite charges. You would not draw separate molecules.

How the marks are awarded

04.1 [1 mark]

One clear definition: a mixture designed as a useful product.

04.2 [4 marks]

1 mark for conversion, 1 for correct substitution, 1 for calculation, 1 for correct final form and sig figs.

04.3 [4 marks]

Marks for: giant lattice, ionic, strong electrostatic forces, and large energy needed to overcome them.

❌ Overall common traps in this question

Mixing up formulation with “compound” or “solution”.
Forgetting to convert kg to g in the moles calculation.
Not using the given Mr = 80.
Explaining the melting point using the wrong type of forces.

Topics

Chemistry · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry · C10: Using Resources