Edexcel A-Level Chemistry Paper 3, November 2020: Question 3

18 marks · Medium difficulty · Calculations

Perform calculations and evaluate titration techniques for a weak acid-strong base titration experiment using sodium hydroxide and ethanoic acid.

Practise this question

Question

A multi-part exam question about a titration experiment involving sodium hydroxide and ethanoic acid. Part (a) asks about washing techniques and excess water in volumetric flasks. Part (b) relates to burette preparation and rinsing effects. Part (c) includes a pH titration curve and questions assessing indicator suitability (methyl red and a table of indicators). Part (d) provides a titration results table to complete and asks for percentage measurement uncertainty. Part (e) gives experimental data and a balanced equation for the reaction between CH3COOH and NaOH, requiring calculation of the concentration of the ethanoic acid solution in g dm^-3.
Question text

3 This question is about a titration experiment carried out by a group of students to

determine the concentration of a solution of ethanoic acid using sodium hydroxide.

(a) A student weighed about 4.00g of sodium hydroxide pellets and added them to a

beaker containing 50 cm3 of deionised water.

The mixture was stirred with a glass rod to dissolve the pellets and to give a

homogenous solution.

The solution was poured through a funnel into a 250.0 cm3 volumetric flask and

deionised water was added up to the mark and then the flask was shaken.

(i) Describe how you would ensure that all the sodium hydroxide was transferred

to the volumetric flask.

(2)

(ii) A student adds deionised water above the mark and shakes the flask.

State why the procedure has to be restarted rather than using a teat pipette

to remove the excess water.

(1)

(b) Two students each cleaned a burette, then poured sodium hydroxide solution into

their burettes.

(i) Student 1 used a funnel to pour sodium hydroxide solution into the burette.

Give two steps needed before the student takes the initial burette reading.

(2)

… *P62670A0632*

(ii) Student 2 cleaned the burette by rinsing it with deionised water immediately

before filling it with the sodium hydroxide solution.

Give the effect, if any, on the value of the first titre. Justify your answer.

(1)

(c) The sketch shows the pH changes during a titration of 25.0 cm3 of ethanoic acid

with sodium hydroxide of the same concentration.

pH

0 10 20 30 40 50

Volume of NaOH(aq) / cm3

The ideal indicator for this titration will change colour on the addition of a very

small volume of sodium hydroxide solution at a titre value very close to the

equivalence point of the reaction.

(i) Assess the suitability of methyl red as an indicator for this titration. Make use 7

of the Data Booklet in answering this question.*P62670A0732*

(4)

(ii) Complete the table, with a tick (*P62670A0832*✓) or a cross (✗), to show whether or not the

indicator would be suitable for use in this titration.

(1)

Indicator pH range Tick or Cross

Bromocresol purple 5.2 – 6.8

Thymol blue 8.0 – 9.6

Thymolphthalein 8.3 – 10.6

Alizarin yellow R 10.1 – 13.0

(d) Each student used a pipette to measure 25.0 cm3 of the ethanoic acid solution

into four separate conical flasks and added an indicator.

The results of one student’s titrations are shown in the table.

Titration number 1 2 3 4

Final burette reading / cm3 13.00 25.50 37.90 50.00

Initial burette reading / cm3 0.25 13.00 25.50 37.90

Titre / cm3

Concordant titres (✓)

(i) Complete the table.

(1)

(ii) The low titre for titration 4 was queried by the teacher. The student had

wanted to refill the burette and continue the titration but had been told the

measurement uncertainty would increase if this was done.

Calculate the total percentage measurement uncertainty if the burette had

been refilled to 0.00, and then a further 0.30 cm3 had been added from the

burette, to the conical flask.

The measurement uncertainty for each burette reading is ±0.05 cm3.

(1)

(e) The teacher carried out the experiment and obtained the following results.

Mass of sodium hydroxide used to make 250.0 cm3 solution = 3.80 g

Volume of ethanoic acid solution = 25.00 cm3

Mean titre of sodium hydroxide = 11.90 cm3

CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

Calculate the concentration of the ethanoic acid solution in g dm−3.

Give your answer to an appropriate number of significant figures.

(5)

*P62670A0932*

(Total for Question 3 = 18 marks)

Mark scheme

Show the mark scheme The mark scheme providing detailed answers and guidance for all parts of Question 3. It outlines specific marking points for volumetric washing techniques, burette checks, pH curve assessment, table completion, percentage uncertainty calculations, and step-by-step mole calculations for the titration concentration determination.

How to answer it

Titration Experiment, pH Curves & Concentration Calculations

What this question tests

This core practical question assesses your mastery of acid-base titrations, standard solution preparation, correct volumetric laboratory techniques, titration curve interpretation, indicator selection criteria, processing titration data (concordancy and uncertainties), and multi-step stoichiometric calculations (converting moles to concentration in g dm⁻³ with correct significant figures).

Part (a) — Preparing a Standard Solution

Transfer Technique and Dilution Errors

✅ Correct Answers

  • (i) Rinse the glass rod, beaker, and funnel with deionized water and transfer all washings into the 250 cm³ volumetric flask.
  • (ii) The total volume of solution would exceed 250 cm³, meaning the concentration would no longer be accurately known.

💡 Key Knowledge

Standard solutions must be prepared rigorously. Any residual solute left on glassware or stirring rods leads to a lower-than-expected concentration unless quantitative washing is performed.

❌ Common Errors

  • Forgetting to wash the funnel or glass rod into the flask.
  • Stating that removing water with a teat pipette "decreases concentration" rather than explaining that the total volume is ruined/unknown.

🧠 Exam Technique

Always link volumetric errors back to either losing solute (lower concentration) or altering the total known volume (unknown concentration).

Part (b) — Burette Preparation and Technique

Burette Setup and Rinsing Rationale

✅ Correct Answers

  • (i) Any two of: Fill the jet space / remove air bubbles; remove the funnel; ensure the burette is vertical; read eyes level with the bottom of the meniscus.
  • (ii) The titre will be larger because residual water in the burette dilutes the NaOH solution, requiring a larger volume to neutralize the acid.

❌ Common Errors

Students often forget to open the tap briefly to fill the jet space below the tap, leaving an air bubble that introduces a systematic error into every titre.

Part (c) — pH Curves and Indicators

Interpreting Titration Curves & Selecting Indicators

✅ Correct Answers

  • (i) Assessment of Methyl Red: The vertical drop/mid-point of the pH curve is around 8.5–8.8, whereas methyl red changes colour around pH 5.1. Its entire pH range does not fall within the vertical region, and its colour change completes long before the equivalence point. Thus, it is unsuitable.
  • (ii) Indicator Table Ticks & Crosses:
    • Bromocresol purple (5.2–6.8): Cross (✗)
    • Thymol blue (8.0–9.6): Tick (✓)
    • Thymolphthalein (8.3–10.6): Tick (✓)
    • Alizarin yellow R (10.1–13.0): Cross (✗)

🧠 Exam Technique

To evaluate an indicator, explicitly compare its narrow colour-change pH range against the steep vertical section (equivalence point) of the specific pH titration curve provided.

Part (d) — Titration Data and Measurement Uncertainty

Processing Titres and Calculating Percentage Uncertainty

✅ Correct Answers

  • (i) Completed Table Titres:
    • Titre 1 = 13.00 - 0.25 = 12.75 cm³ (Not concordant)
    • Titre 2 = 25.50 - 13.00 = 12.50 cm³ (Concordant ✓)
    • Titre 3 = 37.90 - 25.50 = 12.40 cm³ (Concordant ✓)
    • Titre 4 = 50.00 - 37.90 = 12.10 cm³ (Not concordant)

📐 Step-by-Step Calculation (ii)

  1. Identify readings taken: Each titration requires 2 burette readings, plus initial refilling considerations if continued. Total 4 individual burette readings used for titration 4.
  2. Apply uncertainty formula: Percentage Uncertainty = (Total uncertainty ÷ Measured value) × 100
  3. Substitute values: ( (0.05 × 4) ÷ 12.10 ) × 100 = 1.6528%
  4. Final Answer: 1.6% or 1.61% or 2% (Examiner note: do not round 1.65% to 2% incorrectly).
Part (e) — Stoichiometry and Concentration Calculation

Calculating Concentration in g dm⁻³

📐 Step-by-Step Calculation

  1. Moles of NaOH weighed out:
    Molar mass of NaOH = 23.0 + 16.0 + 1.0 = 40.0 g mol⁻¹
    n(NaOH) = 3.80 ÷ 40.0 = 0.0950 mol
  2. Concentration of NaOH solution:
    Volume = 250 cm³ = 0.250 dm³
    [NaOH] = 0.0950 ÷ 0.250 = 0.380 mol dm⁻³
  3. Moles of NaOH in the titration mean titre:
    Mean titre = 11.90 cm³ = 0.01190 dm³
    n(NaOH) = 0.380 × 0.01190 = 0.004522 mol
  4. Moles and molar concentration of CH₃COOH:
    From equation 1:1 ratio, so n(CH₃COOH) = 0.004522 mol in 25.00 cm³ .
    [CH₃COOH] = 0.004522 ÷ 0.02500 = 0.18088 mol dm⁻³
  5. Convert concentration to g dm⁻³ (3 SF):
    Molar mass of CH₃COOH = (12.0 × 2) + (1.0 × 4) + (16.0 × 2) = 60.0 g mol⁻¹
    Concentration in g dm⁻³ = 0.18088 × 60.0 = 10.8528 g dm⁻³
    Final Answer: 10.9 g dm⁻³ (to 3 significant figures).

❌ Common Calculation Traps

  • Forgetting to convert cm³ to dm³ when calculating concentrations and moles.
  • Failing to give the final answer to the requested 3 significant figures.
  • Using titrations 1 or 4 instead of calculating the mean of concordant titres 2 and 3.

💡 Examiner Guidance

Concordant titres are typically within 0.10 cm³ of each other. Always select only concordant values to calculate your mean titre before proceeding with stoichiometry.

Topics

Core Practicals · Physical Chemistry · Core Practical 3: Find the concentration of a solution of hydrochloric acid · Topic 5: Formulae, Equations and Amounts of Substance · Topic 12: Acid-base Equilibria

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.