AQA A-Level Chemistry AS Paper 1, June 2022: Question 2

14 marks · Medium difficulty · Practical Techniques & Data Analysis

Describe standard solution preparation, calculate concentration, identify and correct practical errors in a titration procedure, determine a mean titre, and calculate percentage uncertainty.

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Question text

02 This question is about acid–base titrations.

Citric acid reacts with sodium hydroxide.

C6H8O7(aq) + 3NaOH(aq) → Na3C6H5O7(aq) + 3H2O(l)

02.1 A student makes a solution of citric acid by dissolving some solid citric acid in water.

Describe a method to add an accurately known mass of solid to a beaker to make

a solution.

[2 marks]

02.2 The student dissolves 0.834 g of citric acid in water and makes the solution up to

500 cm3

Calculate the concentration, in mol dm–3, of citric acid in this solution.

[3 marks]

4 –3

Concentration mol dm

02.3 The student uses this method to complete a titration.

• Rinse a burette with distilled water.

• Fill the burette with sodium hydroxide solution.

• Use a measuring cylinder to transfer 25 cm3 of the citric acid solution into a

conical flask.

• Add 5 cm3 of indicator.

*03* • Slowly add the sodium hydroxide solution from the burette into the conical flask.

• Add the sodium hydroxide solution dropwise near the end point until the indicator

just changes colour.

• Repeat the titration to get concordant results.

The method used by the student includes three practical steps that will lead to an

inaccurate final result.

For each of these three steps

• identify the mistake

• explain why it is a mistake

• suggest how the mistake can be overcome.

[6 marks]

02.4 Table 1 shows the student’s burette readings after the mistakes in the practical

procedure have been corrected.

*04* Table 1

Rough Run 1 Run 2 Run 3

Final reading / cm3 23.65 22.95 46.05 26.30

Start reading / cm3 0.00 0.00 22.95 3.40

Titre / cm3 23.65

Complete Table 1.

Use the data in Table 1 to calculate the mean titre.

[2 marks]

Mean titre cm3

02.5 The total uncertainty in the use of the burette is ±0.15 cm3

Calculate the percentage uncertainty in the use of the burette in Run 1.

[1 mark]

Percentage uncertainty

Mark scheme

Show the mark scheme

Question Marking guidance Additional Comments/Guidelines Mark

M1 measure the mass of the weighing boat (or similar) and solid M1 place (an empty) beaker on balance and zero 1

M2 Add the solid to a beaker (or other suitable container) and M2 add the solid to the beaker and record the mass

then reweigh the weighing boat (and subtract to find the mass 1

of solid added.) OR (2 x AO1)

02.1

OR M1 place (an empty) beaker on balance and

measure its mass

M1 Place weighing boat on a balance and zero the balance M2 add the solid to the beaker and subtract mass

M2 Add the solid to a beaker (or other suitable container), wash out of empty beaker from the total mass

weighing boat and transfer washing to the beaker.

M1 Mr citric acid = 192.0 1

M2 Amount of citric acid = Mass / Mr M2 conseq on M1 1

= 0.834 / 192

= 0.0043438 (mol)

02.2 M3 Concentration = moles / volume M3 conseq on M2 1

= 0.0043438 / 0.5 (3 x AO2)

= 0.00869 (mol dm–3)

Alternative Method

M1 Concentration (g/dm3) = 0.834 / 0.50 = 1.668

M2 Mr citric acid = 192.0– HEMISTRY – –

M3 Concentration (mol/dm3) = M1/M2 = 0.00869

This question is marked using levels of response. Refer to the Mark Use best three of these four stages

Scheme Instructions for Examiners for guidance on how to mark this

12 question. Stage 1

Level 3: a. Problem – using a measuring cylinder

Three stages are covered and the explanation of each stage b. Explanation – large uncertainty / not

is generally correct and virtually complete. accurate enough

Answer is well structured with no repetition or irrelevant 5-6 c. Improvement – use a (volumetric)

points. pipette (Not dropping pipette)

Accurate and clear expression of ideas with no errors in use

Stage 2

of technical terms.

a. Problem – too much indicator

Level 2:

b. Explanation – may react and affect the

Three stages are covered but the explanation of each stage

endpoint reading

may be incomplete or may contain inaccuracies OR two

c. Improvement – use a smaller volume

stages are covered and the explanations are generally

(2-6 drops) 6

correct and virtually complete. 3-4

Stage 3

02.3 Answer shows some attempt at structure. a. Problem – rinsing the burette with (3 x

Ideas are expressed with reasonable clarity with, perhaps, distilled or deionised water AO1, 3

some repetition or some irrelevant points. b. Explanation – will slightly dilute the x AO3)

Some minor errors in use of technical terms. alkali solution

Level 1: c. Improvement – rinse the burette with

Two stages are covered but the explanation of each stage alkali solution

may be incomplete or may contain inaccuracies, OR only

one stage is covered but the explanation is generally correct Stage 4

and virtually complete. a. Problem – adding alkali solution until

1-2 the indicator “just” changes colour

Answer includes isolated statements but these are not

presented in a logical order or show some confusion. b. Explanation – acid may not have fully

Answer may contain valid points which are not clearly linked reacted (as mixture not swirled)

to an argument structure. Errors in the use of technical c. Improvement – add alkali solution until

terms. a permanent colour change is seen.

– MISTRY – – JUNE 2022

Level 0

Insufficient correct chemistry to gain a mark. 0

Calculates the titres for each of 1,2,3 as

12 3

02.4 22.95 23.10 22.90

Averages concordant titres:

33 1

(22.95 + 22.90) ÷ 2 = 22.93 cm Allow 22.9(25) cm

(2 x AO1)

(0.15 / 22.95) × 100 = 0.65% 0.15 / (Their Run 1) × 100 1

02.5

(AO1)

How to answer it

Citric Acid Titrations: Quantitative Analysis & Practical Techniques

📌 What This Question Tests

Required Practical 1 Skills & Quantitative Mole Calculations:

Weighing by difference: Technique for transferring a known mass of solid accurately without loss.
Standard solution calculation: Determining molar mass (M_r), amount in moles, and molar concentration in mol dm⁻³.
Evaluating practical titration errors: Apparatus choice (pipette vs cylinder), indicator volume effects, rinsing glassware, and observing persistent end points.
Processing volumetric data: Calculating concordant titres (within 0.10 cm³) and finding their mean.
Uncertainty analysis: Calculating percentage apparatus uncertainty from absolute uncertainty and reading volume.

💡 Chemical Reaction Reference

C₆H₈O₇(aq) + 3 NaOH(aq) → Na₃C₆H₅O₇(aq) + 3 H₂O(l)

Note the 1 : 3 reacting stoichiometric ratio between citric acid (a tribasic/tricarboxylic acid) and sodium hydroxide.

Part 02.1 • 2 Marks

Accurately Adding a Known Mass of Solid

Describing standard laboratory preparation technique

✅ Model Answer (Weighing by Difference)

Step 1 [M1]: Weigh the weighing boat (or container) containing the solid citric acid on a balance.
Step 2 [M2]: Tip the solid into the beaker and reweigh the empty weighing boat, subtracting the final mass from the initial mass to determine the exact mass added.

💡 Alternative Valid Approaches

Tare and washings: Zero (tare) the balance with the weighing boat on it, add solid to obtain mass [M1], transfer to beaker, and wash any residue remaining in the boat into the beaker with distilled water [M2].
Direct beaker weighing: Place the beaker directly on the balance, tare or record empty mass [M1], add the solid directly into the beaker, and record mass added [M2].

❌ Common Errors & Lost Marks

Forgetting reweighing: Saying "weigh 0.834 g into a boat and pour it in." This loses M2 because solid often remains stuck to the weighing boat.
Vagueness: Stating "weigh the boat, empty it" without stating you must reweigh the boat and subtract.

Mark Breakdown: 2 × AO1. M1 awarded for initial mass determination; M2 awarded for ensuring residual mass is accounted for (subtraction or washings).

Part 02.2 • 3 Marks

Calculating Solution Concentration

Finding the concentration of 0.834 g of citric acid in 500 cm³

📐 Step-by-Step Calculation

Step 1: Calculate the relative formula mass (M_r) of citric acid (C₆H₈O₇) [M1]
M_r = (6 × 12.0) + (8 × 1.0) + (7 × 16.0) = 72.0 + 8.0 + 112.0 = 192.0
Step 2: Calculate the amount in moles of citric acid [M2]
Moles = mass / M_r = 0.834 / 192.0 = 0.00434375 mol (or 4.344 × 10⁻³ mol)
Step 3: Convert volume and calculate molar concentration [M3]
Volume = 500 cm³ = 500 / 1000 = 0.500 dm³
Concentration = moles / volume = 0.00434375 / 0.500 = 0.00869 mol dm⁻³ (or 8.69 × 10⁻³ mol dm⁻³)

🧠 Exam Technique: Alternative Pathway

You can find concentration in g dm⁻³ first, then convert:

Concentration in g dm⁻³ = 0.834 g / 0.500 dm³ = 1.668 g dm⁻³
Concentration in mol dm⁻³ = 1.668 / 192.0 = 0.00869 mol dm⁻³

❌ Common Errors

Units conversion fail: Forgetting to divide 500 cm³ by 1000, giving an answer out by a factor of 1000.
M_r calculation slip: Miscounting hydrogen (8) or oxygen (7) atoms.
Rounding too early: Rounding moles to 0.004 causes significant rounding errors in the final concentration.

Mark Breakdown: 3 × AO2. M1 for M_r = 192.0; M2 for moles (consequential on M1); M3 for final concentration = 0.00869 mol dm⁻³.

Part 02.3 • 6 Marks (Levels of Response)

Evaluating Inaccuracies in Titration Procedure

Identify 3 mistakes, explain why each causes error, and provide an improvement

🧠 How Level-of-Response is Graded (6 Marks)

Level 3 (5–6 marks): Covers 3 distinct stages completely (problem identified, accurate explanation, and valid correction). Well-structured, clear chemical language.
Level 2 (3–4 marks): Covers 3 stages with minor gaps/omissions OR covers 2 stages completely.
Level 1 (1–2 marks): Covers 2 stages partially OR covers 1 stage completely.

Stage 1: Measuring Cylinder

Problem: Used a measuring cylinder to transfer 25 cm³ of citric acid.
Explanation: Measuring cylinders have a high percentage uncertainty / are not precise or accurate enough for volumetric analysis.
Improvement: Use a 25 cm³ volumetric pipette (do not just say "dropping pipette").

Stage 2: Excessive Indicator Volume

Problem: Added too much indicator (5 cm³).
Explanation: Indicators are weak acids or weak bases. Adding a large volume will react with the titrant (NaOH) and distort the endpoint reading / titre.
Improvement: Add only 2 to 6 drops (a few drops) of indicator.

Stage 3: Rinsing Burette with Distilled Water

Problem: Burette rinsed only with distilled water before filling with NaOH.
Explanation: Water droplets clinging to the inside dilute the sodium hydroxide solution, leading to an artificially larger titre needed for neutralisation.
Improvement: Rinse the burette with the sodium hydroxide solution (after rinsing with water).

Stage 4 (Alternative): End Point Observation

Problem: Stopping when the indicator "just changes colour" without ensuring complete reaction.
Explanation: Without thorough continuous swirling, localised colour changes occur prematurely before the acid and alkali have fully reacted.
Improvement: Swirl continuously and titrate until a permanent colour change persists.

❌ Common Student Pitfalls on Practical Questions

Vague apparatus names: Writing just "pipette" instead of "volumetric pipette". Dropping pipettes are rejected!
Missing the chemical explanation for indicators: Many students write "it makes it too dark to see". Examiners demand the chemical reason: indicators are weak acids/bases that react and alter the required titre.
Confusing pipette and burette rinsing: Remember: glassware that measures fixed volumes into the conical flask (volumetric pipette) or delivers titrant (burette) MUST be rinsed with the solution they will contain. The conical flask is rinsed with distilled water only!

Mark Breakdown: 6 marks (3 × AO1, 3 × AO3). Structured by 3 stages: each containing (a) Problem, (b) Explanation, (c) Improvement.

Part 02.4 • 2 Marks

Table Completion & Concordant Mean Titre

Processing experimental burette data

📐 1. Complete the Titre Values

Reading	Rough	Run 1	Run 2	Run 3
Final reading / cm³	23.65	22.95	46.05	26.30
Start reading / cm³	0.00	0.00	22.95	3.40
Titre / cm³	23.65	22.95	23.10	22.90

All three calculated titres (22.95, 23.10, 22.90) must be correct to award [M1].

🧠 2. Selecting Concordant Titres [M2]

Concordant titres are results that agree within 0.10 cm³ of each other:

Run 1 = 22.95 cm³
Run 2 = 23.10 cm³ (Difference with Run 1 is 0.15 cm³; with Run 3 is 0.20 cm³ → NOT concordant)
Run 3 = 22.90 cm³ (Difference with Run 1 is only 0.05 cm³ → CONCORDANT!)

Mean titre:
Mean = (22.95 + 22.90) / 2 = 22.93 cm³ (or 22.925 cm³)

❌ Common Calculation Traps

Including the Rough titre: Never include the rough titre when calculating the mean!
Averaging all three accurate runs: Averaging 22.95, 23.10, and 22.90 gives 22.98 cm³ — this scores 0 for M2 because 23.10 cm³ is not concordant with the others.
Inconsistent decimal places: Burette readings must always be recorded to 2 decimal places (ending in .00 or .05).

Mark Breakdown: 2 × AO1. M1 for correctly filling all three titres; M2 for identifying Run 1 and Run 3 as concordant and calculating mean = 22.93 cm³.

Part 02.5 • 1 Mark

Percentage Uncertainty in Run 1

Apparatus error evaluation

📐 Formula & Calculation

Percentage Uncertainty = (Total Uncertainty / Measured Value) × 100

Total uncertainty in burette = ±0.15 cm³ (already incorporates initial and final readings)
Titre for Run 1 = 22.95 cm³
Percentage uncertainty = (0.15 / 22.95) × 100 = 0.65%

❌ Common Misconceptions

Multiplying uncertainty by 2 when already given as total: The question states "The total uncertainty in the use of the burette is ±0.15 cm³". Do not multiply by 2 again!
Dividing by the wrong titre: Ensure you read specifically which run the question asks for (Run 1 = 22.95 cm³).

Mark Breakdown: 1 × AO1. Awarded for 0.65% (consequential on candidate's Run 1 titre).

Topics

Physical Chemistry · Required Practicals · 3.1.2 Amount of Substance · 3.1.12 Acids and Bases · Required Practical 1: Making up a volumetric solution

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.