AQA A-Level Chemistry AS Paper 1, June 2023: Question 3

4 marks · Medium difficulty · Practical Techniques & Data Analysis

Suggest three improvements to a given method for preparing a standard solution, and calculate the percentage uncertainty in the volume of a 250 cm³ volumetric flask.

Practise this question

Question

Question 03 details a six-step method for preparing a standard solution of sodium carbonate using a balance, beaker, and 250 cm³ volumetric flask. Part 03.1 asks to suggest three improvements to this method for 3 marks. Part 03.2 states that the uncertainty for the 250 cm³ volumetric flask is ±0.20 cm³ and asks to calculate the percentage uncertainty in the volume for 1 mark.
Question text

03 A student uses this method to prepare a standard solution of sodium carbonate.

1. Weigh a clean, dry, empty container on a balance that reads to 2 decimal places.

2. Add about 2.5 g of solid sodium carbonate to the container.

3. Tip the solid into a beaker.

4. Add approximately 100 cm3 of distilled water to the beaker and stir until all the solid

has dissolved.

5. Pour the solution into a 250 cm3 volumetric flask.

6. Add distilled water until the top of the meniscus is level with the graduation mark.

03.1 Suggest three improvements to this method.

[3 marks]

03.2 A different student uses the correct method to prepare

250 cm3 of sodium carbonate solution in a volumetric flask.

The uncertainty for the volumetric flask is ±0.20 cm3

Calculate the percentage uncertainty in the volume of this sodium carbonate solution.

[1 mark]

Percentage uncertainty

Mark scheme

Show the mark scheme Mark scheme for 03.1 lists acceptable improvements including: weighing by difference or washing the weighing container, rinsing the beaker and stirring rod and transferring washings into the flask, viewing the bottom of the meniscus on the graduation mark, and inverting/mixing the flask thoroughly (any three, 3 marks). Mark scheme for 03.2 shows the calculation: (0.20 / 250) × 100 = 0.080% for 1 mark.

Question Marking guidance Additional Comments/Guidelines Mark

ANY THREE Ignore apparatus changes

Record all masses (accurately to 2 decimal places)

Weigh by difference / wash the solid from weighing container into the

beaker / add solid directly to volumetric flask (via a funnel) and

dissolve in approximately 100 cm3 of distilled water.

Wash the beaker into the flask after the solution is transferred to the 3

03.1

volumetric flask / wash the stirring rod into the flask after use / wash (3 x AO3)

beaker and transfer washings to the volumetric flask.

(Use a dropper when adding close to the graduation mark to) ensure

the bottom of the meniscus is on the graduation mark

Mix thoroughly the final solution in the volumetric flask / invert the

flask several times (after making the solution up to the graduation

mark).

0.20 1

03.2 × 100 = 0.080 %

250 (AO2)

How to answer it

Preparing a Standard Solution & Apparatus Uncertainty

What this question tests

This question assesses practical mastery of Required Practical 1 (Make up a volumetric solution). You are evaluated on identifying errors and procedural omissions in a standard experimental method, detailing laboratory best practice (weighing by difference, washing techniques, viewing the meniscus, and homogenisation), and calculating apparatus percentage uncertainty.

Question 03.1

Method Improvements for Preparing a Standard Solution

Suggest three improvements to the student's experimental method [3 marks]

✅ Mark Scheme Accepted Answers (Any 3)

  • Weighing by difference: Re-weigh the weighing container after tipping the solid out (or wash residual solid from the container into the beaker).
  • Rinsings / Washings: Rinse the beaker and the glass stirring rod with distilled water and transfer all washings into the volumetric flask.
  • Meniscus alignment: Ensure the bottom of the meniscus rests on the graduation mark (using a dropping pipette near the mark), rather than the top.
  • Inversion / Homogenisation: Stopper and invert the volumetric flask several times to mix thoroughly.
  • Recording data: Explicitly record all mass measurements to 2 decimal places.

💡 Key Practical Steps (RP1)

  • Transfer losses: Solid sticks to weighing boats; dissolved solute coats beaker walls and stirring rods. Quantitative washings eliminate transfer loss.
  • Meniscus rule: Aqueous solutions curve upwards due to surface tension against glass; measurements must always be read at the bottom of the meniscus at eye level.
  • Density differences: Adding water on top creates concentration layers. Without inversion, the concentration is not uniform.

🧠 Exam Technique & Examiner Insights

  • Step-by-step audit: Walk methodically through each numbered step in the stem to spot standard omissions.
    • Steps 2 & 3: Did they weigh by difference? No.
    • Step 4: Was the stirring rod rinsed? No.
    • Step 5: Were beaker washings transferred? No.
    • Step 6: Did they read the bottom of the meniscus? No (it says "top"). Was it inverted? No.
  • Note the restriction: The mark scheme states "Ignore apparatus changes". Suggesting a "3 decimal place balance" or a "larger beaker" scores zero marks. Focus on procedural actions.

❌ Common Student Mistakes

  • Apparatus upgrading: Stating "use a more precise balance" instead of describing procedural improvements.
  • Vague washings: Saying just "clean the beaker" without stating that washings must be transferred into the volumetric flask.
  • Missing the meniscus clue: Overlooking that Step 6 explicitly claims to align the "top of the meniscus".
Mark Allocation: 3 marks total (1 mark for each valid, distinct procedural improvement; AO3 practical evaluation).
Question 03.2

Percentage Uncertainty Calculation

Calculate percentage uncertainty in 250 cm³ volumetric flask (uncertainty ±0.20 cm³) [1 mark]

📐 Step-by-Step Calculation

  1. Recall the formula:
    Percentage uncertainty = (Uncertainty ÷ Measured value) × 100
  2. Substitute the given values:
    = (0.20 ÷ 250) × 100
  3. Calculate final value:
    = 0.080% (or 0.08%)

🧠 Flask vs Burette Uncertainties

  • Single reading vs two readings: A volumetric flask has a single calibration mark, so the uncertainty is applied only once:
    1 × 0.20 cm³ .
  • Contrast this with a burette, where a titre requires two readings (initial and final), meaning the reading uncertainty is multiplied by 2:
    2 × uncertainty .
  • Always check units: both the uncertainty ( 0.20 cm³ ) and the flask volume ( 250 cm³ ) are in cm³, so no unit conversion is required.

❌ Common Calculation Traps

  • Multiplying uncertainty by 2: Incorrectly treating the volumetric flask like a burette titration reading:
    (2 × 0.20 / 250) × 100 = 0.16% (Incorrect).
  • Arithmetic / Power of 10 slip: Forgetting to multiply by 100 and writing 0.0008% .

✅ Final Answer

Percentage uncertainty = 0.080%

(Acceptable: 0.08% or 0.080%)

Mark Allocation: 1 mark (AO2 single-step mathematical calculation).

Topics

Physical Chemistry · Required Practicals · 3.1.2 Amount of Substance · Required Practical 1: Making up a volumetric solution

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.