AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2025: Question 8

11 marks · Standard Demand difficulty · Short Answer

Analyze a practical investigation determining the mass and mean concentration of dissolved solids in different sea water samples.

Practise this question

Question

Question text

08 A student investigated the mass of dissolved solids in sea water.

This is the method used.

1. Weigh an empty evaporating basin.

2. Measure 10.0 cm3 of sea water sample A into the evaporating basin

using a burette.

3. Heat the evaporating basin until all the water seems to have evaporated.

4. Weigh the evaporating basin and contents.

5. Calculate the mass of dissolved solids in the sample.

6. Repeat steps 1 to 5 with sea water samples B, C and D.

08.1 Why did the student use a burette rather than a measuring cylinder to measure

10.0 cm3 of each sample?

[1 mark]

08.2 What was the dependent variable in this investigation?

[1 mark]

08.3 Describe how the student could make sure all of the water had evaporated in step 3.

[2 marks]

08.4 How did the student calculate the mass of dissolved solids in each sea water sample?

[1 mark]

Table 3 shows the results.

Table 3

Sea water sample A B C D

Mass of dissolved solids in grams 0.35 0.37 0.33 0.34

08.5 Calculate the mean concentration of dissolved solids in 10.0 cm3 samples of

sea water A, B, C and D.

Use Table 3 and the equation:

3 mean mass of dissolved solids (g)

mean concentration of dissolved solids (g/dm ) = 3

volume of sample (dm )

Give your answer in g/dm3.

[5 marks]

Mean concentration = g/dm3

08.6 Suggest why sea water samples A, B, C and D contain different masses

of dissolved solids.

[1 mark]

Mark scheme

Show the mark scheme

Question 8

AO /

Question Answers Extra information Mark

Spec. Ref.

08.1 (a burette is) more accurate allow (a burette has) a higher 1 AO3

(than a measuring cylinder) resolution 4.10.1.2

RPA8

ignore precise

AO /

Spec. Ref.

08.2 the mass of (dissolved) solids 1 AO1

4.10.1.2

RPA8

AO /

Spec. Ref.

08.3 reheat (the evaporating basin 1 AO3

and contents) 4.10.1.2

RPA8

(and reweigh) until constant 1

mass

allow for 2 marks

heat to constant mass

AO /

Spec. Ref.

08.4 subtract the mass of the (empty) allow subtract initial mass from 1 AO1

evaporating basin from the the final mass 4.10.1.2

mass of the evaporating basin RPA8

and contents (at the end of the

experiment)

AO /

Spec. Ref.

08.5 (allow mean mass =) (allow mean mass =) 1 AO2

0.35 + 0.37 + 0.33 + 0.34 1.39 4.3.2.5

44 4.10.1.2

RPA8

= 0.3475 (g) 1

(conversion 10 cm3 =) 0.01 dm3 1

0.3475 allow correct use of incorrectly 1

(mean concentration =)

0.01 determined mean mass

allow correct use of incorrect /

no conversion of volume

= 34.75 (g/dm3) allow 34.75 (g/dm3) correctly 1

rounded to at least 2 significant

figures

alternative approach 1

(conversion 10 cm3 =)

0.01 dm3 (1)

(concentrations =)

0.35 0.37 0.33 0.34 allow correct use of incorrect /

(1) no conversion of volume

0.01 0.01 0.01 0.01

= 35 37 33 34 (1)

(mean concentration =) (allow mean concentration =)

35 + 37 + 33 + 34 139

(1)

allow correct use of incorrectly

determined concentrations

= 34.75 (g/dm3) (1) allow 34.75 (g/dm3) correctly

rounded to at least 2 significant

figures

alternative approach 2 using

total mass and total volume

(total mass =

0.35 + 0.37 + 0.33 + 0.34 =)

1.39 (g) (1)

(total volume =

10 + 10 + 10 + 10 =)

40 cm3 (1)

(conversion 40 cm3 =)

0.04 dm3 (1)

1.39 allow correct use of incorrectly

(mean concentration =)

0.04 determined total mass

allow correct use of incorrect /

no conversion of total volume

= 34.75 (g/dm3) (1) allow 34.75 (g/dm3) correctly

rounded to at least 2 significant

figures

AO /

Spec. Ref.

08.6 the samples were taken from 1 AO3

different places 4.10.1.2

Total Question 8 11

How to answer it

Determining Dissolved Solids in Sea Water (RPA 8)

📌 What This Question Tests

Core Knowledge & Practical Skills (AQA RPA 8: Water Purification):

Apparatus & Accuracy: Comparing volumetric glassware (burette vs measuring cylinder) and understanding resolution.
Scientific Variables: Identifying the dependent variable from an experimental procedure.
Experimental Validity: Knowing the standard method for heating to constant mass to ensure complete dryness.
Data Handling & Tare Mass: Calculating mass of residue by difference.
Quantitative Chemistry: Multi-step concentration calculation ( g/dm³ ), including finding a mean and unit conversion ( cm³ to dm³ ).
Scientific Reasoning: Explaining variation in real-world environmental samples.

Question 08.1

Burette vs. Measuring Cylinder

Apparatus selection • 1 Mark

✅ Correct Answers

(A burette is) more accurate (than a measuring cylinder).
OR: (A burette has) a higher resolution.

❌ Common Errors & Misconceptions

Writing "it is more precise" — the mark scheme explicitly says ignore precise. Precision relates to repeatability, not the instrument's scale intervals.
Writing vague phrases like "it is easier to read" or "less error" without naming accuracy or resolution.

Mark Breakdown: [1 mark] for stating that the burette provides higher accuracy or smaller scale divisions (higher resolution).

Question 08.2

Identifying the Dependent Variable

Working scientifically: Variables • 1 Mark

✅ Correct Answer

The mass of (dissolved) solids

🧠 Exam Technique

Remember the golden rule of variables:

Independent variable: What you change (the sea water sample: A, B, C, D).
Dependent variable: What you measure as a result (the mass of dissolved solid).
Control variable: What you keep the same (volume of sample = 10.0 cm³).

Mark Breakdown: [1 mark] for identifying mass of dissolved solids (or mass of residue/salt).

Question 08.3

Ensuring Complete Evaporation

Experimental procedure • 2 Marks

✅ Correct Answer

Reheat the evaporating basin and contents [1 mark]
(and reweigh) until constant mass is reached [1 mark]

Note: Writing "heat to constant mass" awards both marks directly.

💡 Key Knowledge: "Heating to Constant Mass"

This is a classic required practical technique. You cannot tell if a solid is dry simply by looking at it:

Heat sample → cool → weigh.
Heat again → cool → reweigh.
If mass decreases, water was still present.
Repeat until two consecutive readings are identical.

Mark Breakdown: [1 mark] for reheat basin/contents; [1 mark] for weighing until mass no longer changes (constant mass).

Question 08.4

Calculating Mass of Dissolved Solids

Experimental calculation by difference • 1 Mark

✅ Correct Answer

Subtract the mass of the (empty) evaporating basin from the mass of the evaporating basin and contents (at the end of the experiment).

Also allowed: "Subtract initial mass from the final mass".

🧠 Exam Technique: Be Specific

Refer directly to the numbered steps in the method:

Mass of solid = (Mass from Step 4) − (Mass from Step 1)

Avoid saying simply "weigh it" — you cannot scrape all the dry salt out to weigh it alone!

Mark Breakdown: [1 mark] for clear statement of subtracting empty basin mass from total final mass.

Question 08.5

Mean Concentration Calculation

Quantitative chemistry • 5 Marks

📐 Step-by-Step Calculation

Given Equation: mean concentration (g/dm³) = mean mass of dissolved solids (g) ÷ volume of sample (dm³)

1 Calculate the mean mass of dissolved solids:

Total mass = 0.35 + 0.37 + 0.33 + 0.34 = 1.39 g

Mean mass = 1.39 ÷ 4 = 0.3475 g [Mark 1 & Mark 2]

2 Convert volume from cm³ to dm³:

There are 1000 cm³ in 1 dm³, so divide by 1000:

Volume = 10.0 cm³ ÷ 1000 = 0.01 dm³ [Mark 3]

3 Substitute into the concentration equation:

Mean concentration = 0.3475 g ÷ 0.01 dm³ [Mark 4]

4 Final Answer:

Mean concentration = 34.75 g/dm³ [Mark 5]

Note: Rounded values to at least 2 significant figures (e.g. 34.8 or 35) are also accepted.

❌ Common Traps to Avoid

Forgetting unit conversion: Dividing 0.3475 ÷ 10 gives 0.03475 . You must convert cm³ to dm³!
Dividing by the wrong number of samples: Make sure you divide by 4 (samples A, B, C, D).
Rounding too early: Do not round 0.3475 to 0.35 halfway through the calculation.

🧠 Error Carried Forward (ECF)

Even if you made an arithmetic slip finding the mean, full method marks are awarded if you correctly divide your mean mass by 0.01 dm³ .

Alternative valid method: Find each concentration first ( 35, 37, 33, 34 g/dm³ ) and calculate their mean → gives the identical result 34.75 g/dm³ .

Mark Breakdown (5 Marks total):
• Mark 1: Addition of all 4 masses and dividing by 4.
• Mark 2: Correct mean mass = 0.3475 (g).
• Mark 3: Volume conversion: 10 cm³ = 0.01 dm³.
• Mark 4: Correct substitution: mean mass ÷ 0.01.
• Mark 5: Final value = 34.75 (g/dm³).

Question 08.6

Variation in Sea Water Samples

Application of knowledge • 1 Mark

✅ Correct Answer

The samples were taken from different places / locations.

💡 Examiner Insight

In nature, salinity varies depending on distance from rivers (freshwater runoff dilution), depth of water, local rate of evaporation, or weather conditions.
Stating that the samples came from different locations / different depths / different sources earns the mark immediately.

Mark Breakdown: [1 mark] for suggesting samples came from different locations/sources.

Topics

Chemistry · Required Practicals · Required Practicals · C10: Using Resources · C3: Quantitative Chemistry

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.