AQA AS Level Biology Paper 2, June 2023: Question 8
8 marks · Medium difficulty · Practical Techniques & Data Analysis
Investigate protein concentration using copper sulfate solution densities, dilution series preparation, graph interpretation, and evaluate consequences of blood testing errors.
Practise this questionQuestion
Question text
08.1 A student investigated a method for estimating the concentration of protein in solution
by using a measure of the density of the solutions.
Copper sulfate solutions of different concentration have known densities, so they can
be used to measure the density of other solutions.
The student prepared a dilution series of a copper sulfate solution.
Complete by giving all headings, units and volumes required to make 30 cm3
Table 3
of the concentration of the copper sulfate solution shown.
[2 marks]
Table 3
Concentration of copper Volume of 100 g kg–1 copper
Volume of water /
sulfate solution / sulfate solution /
g kg–1
08.2 Table 4 shows the densities of the dilution series of the copper sulfate solution.
Table 4
Concentration of copper –3
–1 Density of solution / g cm
sulfate solution / g kg
0 0.997
25 1.014
50 1.030
75 1.048
100 1.065
Figure 5 shows the densities of protein solutions of different concentration.
Figure 5
The student put one drop of 10% protein solution into each of the copper sulfate
solutions shown in Table 4.
Using Figure 5, he predicted that the drop would sink in the 0 and 25 g kg–1 copper
sulfate solutions and float in the 50, 75 and 100 g kg–1 copper sulfate solutions.
Give the density of the 10% protein solution and explain why the student predicted
that the drop would sink in the 25 g kg–1 copper sulfate solution.
[2 marks]
Density of 10% protein solution g cm–3
Explanation
08 3 State the range of possible concentrations of a protein solution that sinks in 75 g kg–1
.
copper sulfate solution and floats in 100 g kg–1 copper sulfate solution.
[1 mark]
Minimum concentration %
Maximum concentration %
08.4 Blood donation involves healthy donors giving blood that can be used to treat
hospital patients.
When donors arrive, the haemoglobin concentration of their blood is tested.
A sample of each donor’s blood is added to a copper sulfate solution to determine
whether the haemoglobin concentration is high enough to donate.
Errors sometimes occur with this test.
Tom has a concentration of haemoglobin high enough to donate.
Lucy has a concentration of haemoglobin too low to donate.
Evaluate the consequences of errors occurring when Tom’s and Lucy’s blood samples
are tested.
[3 marks]
Consequences of measurement error for Tom’s blood
Consequences of measurement error for Lucy’s blood
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
One mark for each row. Accept dm3 / mm3 for
volume unit.
If values do not match the given unit, max 1.
Accept 0.0225/2.25 ×
Concentration Volume of 100 10–2/22 500 and
of copper g kg–1 0.0075/7.5 ×
sulfate copper sulfate Volume of 2 10–3/7500
08.1 solution / solution / water / (2 x
Ignore units in 2nd
–1 AO3) row.
g kg 3 3
cm cm
Do not accept
mm–3/cm–3/dm–3/ml
22.5 7.5
1. Density of 10% protein solution = 1.028; 2
08.2 2. More dense (than 25 g kg–1 copper sulfate (2 x
solution); AO2)
Must be in correct
16.5 and 22; 1
08.3 order
(AO3)
Any three from:
(Tom) Max 2 for either Tom
or Lucy
1. (Healthy donor) not allowed to donate;
2. Less blood collected
If no credit awarded,
OR
max 1 mark for idea
Fewer patients treated; 3 max of too little
08.4 (3 x haemoglobin left to
3. Cause Tom anxiety (about his health);
AO3) carry oxygen in blood
(Lucy) OR
4. (Gives blood) when it may not be safe (for her) to reduced oxygen to
do so; respiring tissues
5. (Her blood) may not help patients;
6. Her (missed) low haemoglobin goes untreated;
How to answer it
Study Guide: Estimating Protein Concentration & Blood Testing
What this question tests
This question assesses your ability to calculate dilution series volumes, interpret graphical data relating density and concentration, apply floating/sinking principles to determine concentration ranges, and evaluate the ethical and physiological consequences of measurement errors in medical screening contexts like blood donation.
Dilution Series Calculation
✅ Correct Answer
Volume of 100 g kg⁻¹ copper sulfate solution: 22.5 cm³
Volume of water: 7.5 cm³
Total: 2 marks (1 mark per correct row entry with correct units)
📐 Step-by-Step Calculation
- Identify target concentration: 75 g kg⁻¹ from a stock of 100 g kg⁻¹ with a final volume of 30 cm³.
- Use dilution factor: Target/Stock = 75 / 100 = 0.75 (or 3/4).
- Calculate stock volume: 30 cm³ × 0.75 = 22.5 cm³ .
- Calculate water volume: Total volume minus stock volume = 30 cm³ - 22.5 cm³ = 7.5 cm³ .
❌ Common Errors
- Failing to include units or writing incorrect volume units (e.g., using mm⁻³ or ml instead of cm³ or dm³ ).
- Inverting the ratios and mixing up the volume of stock solution with the volume of water.
Density Interpretation and Prediction
✅ Correct Answer
Density of 10% protein solution: 1.028 g cm⁻³ (accept 1.027 – 1.029)
Explanation: The drop of protein solution is more dense than the 25 g kg⁻¹ copper sulfate solution (which has a density of 1.014 g cm⁻³).
Total: 2 marks
💡 Key Knowledge
An object sinks if its density is greater than the density of the surrounding fluid, and floats if its density is less than the fluid density.
🧠 Exam Technique
When reading interpolation graphs like Figure 5, use a sharp pencil and ruler to line up the axes carefully. Cross-reference the density value obtained (1.028) against Table 4 densities to explain sinking/floating behavior.
Determining Concentration Ranges
✅ Correct Answer
Minimum concentration: 16.5 %
Maximum concentration: 22 %
Total: 1 mark (must be in the correct order)
💡 Key Knowledge
Sinking in 75 g kg⁻¹ solution (density 1.048) means the protein density is greater than 1.048 g cm⁻³, corresponding to a protein concentration of ~16.5%. Floating in 100 g kg⁻¹ solution (density 1.065) means density is less than 1.065 g cm⁻³, corresponding to ~22% protein concentration on Figure 5.
Evaluating Measurement Errors in Blood Donation
✅ Correct Answer (Any 3 points)
For Tom (false negative / error):
- Healthy donor not allowed to donate blood.
- Less blood collected / fewer patients treated.
- Causes Tom unnecessary anxiety about his health.
For Lucy (false positive / error):
- Gives blood when it may not be safe for her to do so.
- Her blood may not help patients effectively.
- Her low haemoglobin condition goes untreated.
Total: 3 max marks
🧠 Exam Technique
To score full marks in "evaluate consequences" questions, you must address both individuals mentioned in the prompt. Ensure you cover physiological impacts (e.g., oxygen transport, untreated anaemia) as well as operational impacts (blood supply volume).
❌ Common Errors
- Focusing only on Tom or only on Lucy, which caps your maximum score at 2 marks.
- Vague statements like "it affects health" without explaining why low haemoglobin matters (reduced oxygen transport to respiring tissues).
Topics
Biology · Practical skills · 3.1 Biological molecules · Experimental design · Data analysis
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.