AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2022: Question 6
8 marks · Standard Demand difficulty · Short Answer
Analyze the electrolysis of potassium sulfate, including chemical formula determination, gas volume measurement, ratio explanation, uncertainty calculation, and concentration calculation.
Practise this questionQuestion
Question text
06 This question is about electrolysis.
Figure 14 shows the apparatus used to investigate the electrolysis of
potassium sulfate solution.
Figure 14
06.1 Potassium sulfate contains K+ and SO 2– ions.
What is the formula of potassium sulfate?
[1 mark]
Tick ( ) one box.
KSO4
K2SO4
K(SO4)2
K2(SO4)2 25
06.2 What are the volumes of gases collected in the electrolysis experiment?
Use Figure 14.
[1 mark]
Volume of hydrogen = cm3
Volume of oxygen = cm3
06.3 A student made the following hypothesis:
‘The volumes of gases collected in this electrolysis experiment are in the same ratio
as hydrogen atoms to oxygen atoms in a water molecule.’
Explain how the volumes of gases collected in the experiment in Figure 14 support
the student’s hypothesis.
Use your answer to Question 06.2
[2 marks]
06.4 The experiment is repeated 4 times.
The volumes of oxygen collected in the 4 experiments are:
6 cm3 9 cm3 10 cm3 11 cm3
The mean volume of oxygen collected in the 4 experiments is 9 cm3
The measure of uncertainty is the range of a set of measurements about the mean.
What is the measure of uncertainty in the 4 experiments?
[1 mark]
Tick ( ) one box.
9 ± 1 cm3
*25* 9 ± 2 cm3
9 ± 3 cm3
06.5 The potassium sulfate solution has 0.86 g of potassium sulfate dissolved in
25 cm3 of water.
Calculate the mass of potassium sulfate needed to make 1.0 dm3 of solution.
[3 marks]
Mass = g
Mark scheme
Show the mark scheme
Question 6
AO /
Question Answers Extra information Mark
Spec. Ref.
06.1 K2SO4 1 AO2
5.1.1.1
5.4.3.4
RPA9
AO /
Spec. Ref.
06.2 (volume of hydrogen) 30 (cm3) 1 AO2
and 5.4.3.4
(volume of oxygen) 15 (cm3) RPA9
AO /
Spec. Ref.
06.3 (because) the ratio of volume of 1 AO3
hydrogen : oxygen is 2 : 1 5.4.3.4
RPA9
(and this is the) same as the ratio of 1
hydrogen (atoms) : oxygen (atoms)
in (formula of) H2O
OR
(because) the ratio of volume of must relate to the volumes
hydrogen : oxygen is not 2 : 1 (1) given in question 06.2
(and this is) different to the ratio of
hydrogen (atoms) : oxygen (atoms)
in (formula of) H2O (1)
AO /
Spec. Ref.
06.4 9 ± 3 cm3 1 AO2
5.3.1.4
AO /
Spec. Ref.
06.5 (conversion) AO2
25 3 5.3.2.5
( = ) 0.025 (dm ) 1
1000 5.4.3.4
(concentration =) allow correct use of incorrect / 1
0.86 no conversion
0.025
= 34.4 (g per dm3) allow 34 (g per dm3) 1
OR
(conversion)
1000
(1)
= 40 (1)
(40 × 0.86) allow correct use of incorrect /
= 34.4 (g per dm3) (1) no conversion
allow 34 (g per dm3)
OR
(concentration =)
0.86
(1)
= 0.0344 (1)
(conversion)
(0.0344 × 1000)
= 34.4 (g per dm3) (1) allow 34 (g per dm3)
Total Question 6 8
How to answer it
Electrolysis of Solutions & Concentration Calculations
What this question tests
This question assesses your ability to derive chemical formulas from ions, interpret experimental data from electrolysis (specifically the decomposition of water), calculate experimental uncertainty, and perform concentration-to-mass conversions.
Determining Ionic Formulas
Key Knowledge
Ionic compounds are neutral. The total positive charge must equal the total negative charge.
- Potassium ion: K⁺ (+1)
- Sulfate ion: SO₄²⁻ (-2)
Correct Answer
K₂SO₄
1 Mark: For selecting the correct formula box.
Exam Technique
To balance the charges, you need two K⁺ ions (2 × +1 = +2) to balance one SO₄²⁻ ion (-2). This gives the subscript "2" after the K.
Reading Experimental Scales
Correct Answer
- Volume of hydrogen = 30 cm³
- Volume of oxygen = 15 cm³
How to read Figure 14
The gas collects at the top of the measuring cylinders, pushing the liquid down. Look at the level of the liquid (the meniscus) against the scale. The hydrogen cylinder shows the level at the 30 mark; the oxygen cylinder shows it at the 15 mark.
Evaluating a Hypothesis
Key Knowledge
In the electrolysis of aqueous potassium sulfate, water (H₂O) is decomposed into hydrogen and oxygen gas. The balanced equation is:
2H₂O → 2H₂ + O₂
Model Answer
The ratio of hydrogen to oxygen volume is 2:1 (30:15). This is the same as the ratio of hydrogen atoms to oxygen atoms in a water molecule (H₂O).
1 Mark: Identifying the 2:1 ratio from your data.
1 Mark: Linking this to the formula of water.
Common Error
Students often forget to use their specific numbers from 06.2. Even if your readings were slightly wrong, you can still get marks here if you correctly explain the ratio of your numbers.
Calculating Uncertainty
Step-by-Step
- Find the Mean: Given as 9 cm³.
- Find the Range: The values are 6, 9, 10, 11.
- Calculate Deviation: How far is the furthest value from the mean?
9 - 6 = 3
11 - 9 = 2 - Result: The largest deviation is 3.
Correct Answer
9 ± 3 cm³
1 Mark: For selecting the correct uncertainty box.
Mass and Concentration
Calculate the mass needed for 1.0 dm³ of solution.
Calculation Steps
Method: Scaling up by Volume
- Identify the relationship: We have 0.86 g in 25 cm³. We want the mass in 1000 cm³ (which is 1.0 dm³).
- Find the scale factor: 1000 / 25 = 40. (The target volume is 40 times larger).
- Multiply the mass: 0.86 g × 40 = 34.4 g.
Final Answer
Mass = 34.4 g
1 Mark: Correct volume conversion (25/1000).
1 Mark: Correct substitution into concentration formula.
1 Mark: Final correct value.
Calculation Traps
- Unit Confusion: Always remember that 1 dm³ = 1000 cm³.
- Significant Figures: The mark scheme allows "34", but "34.4" is more precise based on the data provided.
- Inversion: Don't divide the volume by the mass; concentration is Mass / Volume.
Topics
Chemistry · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Foundation), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.