AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2023: Question 7

10 marks · Standard Demand difficulty · Short Answer

Identify the reaction type and perform quantitative calculations for relative formula mass, percentage by mass, and solution concentration.

Practise this question

Question

The image shows a multi-part chemistry exam question numbered 07. It starts with the chemical equation: 2 LiOH + H2SO4 → Li2SO4 + 2 H2O. Part 07.1 asks for the reaction type. Part 07.2 asks for the relative formula mass (Mr) of sulfuric acid (H2SO4) given relative atomic masses (Ar) for H=1, O=16, and S=32. Part 07.3 asks for the percentage by mass of oxygen in lithium sulfate (Li2SO4) to 2 significant figures, providing Ar of O=16 and Mr of Li2SO4=110. Part 07.4 asks for the concentration in g/dm³ of a solution containing 0.30 g of lithium sulfate in 25 cm³.
Question text

07 Lithium hydroxide reacts with sulfuric acid to produce lithium sulfate.

The equation for the reaction is:

2 LiOH + H2SO4 → Li2SO4 + 2 H2O

07.1 What type of reaction is this?

[1 mark]

07.2 Calculate the relative formula mass (Mr) of sulfuric acid (H2SO4).

Relative atomic masses (Ar): H = 1 O = 16 S = 32

[2 marks]

Relative formula mass (33M ) =

r

07.3 Calculate the percentage by mass of oxygen in lithium sulfate (Li2SO4).

Relative atomic mass (Ar): O = 16

Relative formula mass (Mr): Li2SO4 = 110

Give your answer to 2 significant figures.

[4 marks]

Percentage by mass of oxygen (2 significant figures) = %

07.4 A solution of lithium sulfate contains 0.30 g of lithium sulfate in 25 cm3.

Calculate the concentration of lithium sulfate in g/dm3.

[3 marks]

Concentration = g/dm3

Mark scheme

Show the mark scheme The mark scheme table for question 7 shows the answers and marking points. 07.1 accepts 'neutralisation' (allow exothermic) for 1 mark. 07.2 awards 2 marks for calculating Mr as 98 (1x2 + 32 + 4x16). 07.3 awards 4 marks for calculating percentage of oxygen: first finding mass of oxygen (4 x 16 = 64), then (64 / 110) * 100 = 58.18, and finally rounding to 58%. 07.4 awards 3 marks for concentration: converting 25 cm³ to 0.025 dm³ and then dividing 0.30 g by 0.025 dm³ to get 12 g/dm³.

Question 7

AO /

Question Answers Extra information Mark

Spec. Ref.

07.1 neutralisation allow exothermic 1 AO1

5.4.2.2

RPA8

AO /

Spec. Ref.

07.2 (Mr =) AO2

(1 × 2) + 32 + (4 × 16) 1 5.3.1.2

= 98 1

AO /

Spec. Ref.

07.3 (Ar O × 4 =) AO2

4 × 16 1 5.3.1.2

or

(percentage of oxygen =)

64 allow correct use of incorrectly 1

×100 determined mass of oxygen

= 58.18 1

= 58 (%) allow a correctly calculated 1

answer to 2 significant figures

from an incorrect calculation

which uses the values in the

question

AO /

Spec. Ref.

07.4 (unit conversion) AO2

(25 cm3 ÷ 1000) = 0.025 dm3 1 5.3.2.5

0.30

(conc =) (g/dm3) allow correct use of incorrect / 1

0.025 no unit conversion

= 12 (g/dm3)

alternative approach:

0.30

(g/cm3) (1)

= 0.012 (g/cm3) (1)

(unit conversion)

(0.012 × 1000) allow correct conversion of an

= 12 (g/dm3) (1) incorrect concentration

calculation

Total Question 7 10

How to answer it

Lithium Hydroxide and Sulfuric Acid Calculations

What this question tests

This question assesses your ability to identify chemical reaction types and perform fundamental quantitative chemistry calculations. Key skills include calculating Relative Formula Mass (Mᵣ), determining percentage composition by mass, and calculating concentrations with unit conversions.

Part 07.1

Identifying Reaction Types

Question: What type of reaction is this?

Correct Answer

Neutralisation

1 Mark: Awarded for the correct term.

Key Knowledge

  • Acid + Alkali → Salt + Water
  • Lithium hydroxide (LiOH) is an alkali.
  • Sulfuric acid (H₂SO₄) is an acid.
  • The reaction produces water (H₂O).

Exam Technique

Look at the reactants and products. If you see an acid reacting with a hydroxide to make water, it is always neutralisation. The examiner also accepts "exothermic" as these reactions release heat.

Part 07.2

Relative Formula Mass (Mᵣ)

Question: Calculate the Mᵣ of sulfuric acid (H₂SO₄).

Step-by-Step Calculation

  1. Identify the atoms: 2 × H, 1 × S, 4 × O
  2. Multiply by Aᵣ: (2 × 1) + (1 × 32) + (4 × 16)
  3. Sum the totals: 2 + 32 + 64 = 98

Common Errors

  • Ignoring the subscripts (e.g., calculating H + S + O).
  • Using the Atomic Number instead of the Relative Atomic Mass (Aᵣ).
2 Marks: 1 for the correct working/addition, 1 for the final answer of 98.
Part 07.3

Percentage by Mass

Question: Calculate the percentage by mass of oxygen in Li₂SO₄.

Calculation Steps

  1. Find total mass of Oxygen: 4 × 16 = 64
  2. Divide by total Mᵣ: 64 / 110 = 0.5818...
  3. Multiply by 100: 58.18...%
  4. Round to 2 sig figs: 58%

Significant Figures

The question explicitly asks for 2 significant figures. Even if your calculation is perfect, you will lose the final mark if you write 58.18% or 58.2%.

4 Marks: 1 for mass of oxygen (64), 1 for the fraction (64/110), 1 for the calculation, 1 for rounding to 2 sig figs.
Part 07.4

Concentration Calculations

Question: Calculate the concentration in g/dm³.

Step-by-Step Calculation

  1. Convert Volume: 25 cm³ ÷ 1000 = 0.025 dm³
  2. Formula: Concentration = Mass / Volume
  3. Calculate: 0.30 g / 0.025 dm³ = 12 g/dm³

The "Unit Trap"

The most common mistake is forgetting to convert cm³ to dm³. If you calculate 0.30 / 25 = 0.012, you have calculated g/cm³, not g/dm³.

Examiner Insight

Top-level responses always show the conversion step clearly. If you make a mistake in the division but showed the ÷ 1000 step, you can still earn "Error Carried Forward" marks. Always write down your conversion!

3 Marks: 1 for unit conversion, 1 for the division setup, 1 for the final answer 12.

Topics

Chemistry · C3: Quantitative Chemistry · C4: Chemical Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.