Edexcel A-Level Chemistry AS Paper 1, June 2017: Question 7

9 marks · Medium difficulty · Calculations

Complete a titration results table, calculate the concentration of hydrochloric acid from neutralisation data, identify indicator colour changes, and explain experimental techniques for accurate titration endpoint determination.

Practise this question

Question

Exam question 7 detailing a neutralisation titration of hydrochloric acid with sodium carbonate. Includes a titration table with 5 runs of initial and final burette readings, followed by parts (a) completing the table and calculating concentration (5 marks), (b) multiple choice question on methyl orange colour change (1 mark), and (c) explaining three actions near the endpoint for accuracy (3 marks).
Question text

7 Hydrochloric acid is prepared by dissolving hydrogen chloride gas in water. It is

difficult to dissolve a known amount of hydrogen chloride, so the exact concentration

of such solutions is uncertain. A solution of hydrochloric acid of concentration

between 0.095 mol dm–3 and 0.105 mol dm–3 was prepared.

Before a class attempted a practical using this solution, a technician standardised the

hydrochloric acid with sodium carbonate solution. The technician dissolved 1.30 g of

anhydrous sodium carbonate in water and made up the solution to 100 cm3.

The equation for the reaction which occurs is shown.

Na2CO3 + 2HCl o 2NaCl + H2O + CO2

A 10.0 cm3 portion of the sodium carbonate solution was transferred to a conical flask.

Three drops of methyl orange indicator were added and the solution titrated with

hydrochloric acid. The results for the experiment are shown.

Titration 1 2 3 4 5

Final burette reading / cm3 26.00 34.00 36.10 24.15 48.20

Initial burette reading / cm3 0.00 10.00 11.00 0.05 24.15

Titre / cm3

Concordant results (3)

(a) Complete the table and determine the concentration, in mol dm–3 , of the

hydrochloric acid solution, giving the answer to an appropriate number of

significant figures.

(5)

(b) The colour change at the end-point when methyl orange is used as an indicator

for this titration is from

(1)

A orange to yellow

B red to orange

C yellow to orange

D yellow to red

(c) Explain three actions the technician might take in the procedure, just before

the end-point of the titration, to ensure that the volume of acid added at the

end-point is accurate.

(3)

… *P49858A01824*

(Total for Question 7 = 9 marks)

Mark scheme

Show the mark scheme Mark scheme for question 7 showing the completed titration table, mean titre calculation of 24.05 cm3, step-by-step calculation leading to a concentration of 0.102 mol dm-3 for part (a). Part (b) indicates C is the correct answer. Part (c) lists acceptable points such as adding drop by drop, swirling the flask, rinsing sides with distilled water, or using a white tile.

Question

Acceptable Answer Additional Guidance Mark

Number

7 (a) (5)

Titration 1 2 3 4 5

Total

3 26.00 24.00 25.10 24.10 24.05

Titre /cm Allow lack of second decimal place 0, on

one number only (as a slip)

Concordant

results ( )

Correct total titres (1)

Example of Calculation

Correct 3 ticks and mean titre = 24.05 (cm3) (1)

Moles of sodium carbonate in 100 cm3 =

Moles of sodium carbonate in 100 cm3 1.30/106 = 0.012264151 (mol)

AND

3 So moles of sodium carbonate in 10 cm3 =

moles of sodium carbonate in 10 cm (1)

0.012264151/10

=0.0012264151 (mol)

Moles of HCl (1) Moles of HCl = 0.0012264151 x 2

= 0.0024528302

concentration of HCl (1) So concentration of HCl =

0.0024528302 / (24.05/1000) =

0.1019887

= 0.102 (mol dm-3)(to 3 s.f.)

Allow 0.10 provided working given which

is equal to 0.10 to 2 s.f.

Correct final answer with alternative

route scores 3 marks

Allow TE on incorrect mean titre

Question

Answer Mark

Number

7 (b) 7(b). The only correct answer is C (1)

A is not correct because this is the reverse of the correct colour change

B is not correct because this is doing the reverse titration (acid in flask and carbonate in burette)

D is not correct because this is going beyond the endpoint to an acidic solution

Question

Acceptable Answer Additional Guidance Mark

Number

7 (c) An explanation that makes reference to the following two If no other mark awarded, award 1 for (3)

points: three correct actions

Add drop by drop

AND

So that too much acid is not added / to avoid 'overshooting' Do not award to make the reading

the end-point (1) more precise / accurate (as this is

given in the question)

Swirl / shake / agitate

AND

To ensure a homogenous mixture/allow all acid and alkali to

react (1)

Any one from: Do not award

ensuring the burette is vertical

Rinse the sides of the flask with distilled/deionised water measuring to the bottom of the

between additions meniscus

AND parallax errors

To rinse all reactants into the solution so all can react reducing the speed of the titration

fewer drops of indicator

OR

Use a white tile/paper

AND

To clearly see change of colour

OR

Compare the colour of the solution at the end-point with

previous titrations

AND

To ensure consistency of end-point colour

OR

Rinse the end of the jet of the burette (with distilled water)

AND

To ensure all the hydrochloric acid is in the conical flask (no

drip left on burette (1)

(Total for Question 7 = 9 marks)

How to answer it

Neutralisation Titrations and Practical Technique Study Guide

What this question tests

This multi-step question assesses core AS Chemistry practical skills: processing titration data to find concordant titres, executing stoichiometry calculations with moles and concentrations, identifying indicators and their colour changes, and detailing precision techniques used during titrations to avoid overshooting.

Question 7(a) — Titration Calculation

Processing Titre Data and Calculating Concentration

✅ Completed Titration Table & Mean

Titres (cm³):

  • Titration 1: 26.00
  • Titration 2: 24.00 (omit - rough)
  • Titration 3: 25.10
  • Titration 4: 24.10 (tick)
  • Titration 5: 24.05 (tick)

Concordant mean titre: (24.00 + 24.10 + 24.05) / 3 = 24.05 cm³ (Note: Titrations 2, 4, and 5 are within 0.10 cm³ of each other).

💡 Key Knowledge: Titration Logic

  • Always subtract initial burette reading from final reading to find each titre.
  • Concordant results are titres that agree closely (typically within 0.10 cm³). Only average concordant titres (usually 2 or 3).
  • Always give final concentrations to an appropriate number of significant figures (usually 3 s.f. matching data).

📐 Step-by-Step Calculation

  1. Moles of Na₂CO₃ in 100 cm³:
    Molar mass of Na₂CO₃ = (23.0 × 2) + 12.0 + (16.0 × 3) = 106.0 g mol⁻¹ .
    Moles = mass / molar mass = 1.30 / 106.0 = 0.01226 mol .
  2. Moles of Na₂CO₃ in the 10.0 cm³ pipetted portion:
    0.01226 × (10.0 / 100.0) = 0.001226 mol .
  3. Moles of HCl reacting (using stoichiometry 1:2):
    From equation Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ , moles of HCl = moles of Na₂CO₃ × 2.
    0.001226 × 2 = 0.002453 mol .
  4. Concentration of HCl:
    Concentration = (moles / volume in cm³) × 1000 = (0.002453 / 24.05) × 1000 = 0.102 mol dm⁻³ (to 3 s.f.).

❌ Common Calculation Traps

  • Molar Mass Errors: Forgetting to multiply Na by 2 when calculating the molar mass of sodium carbonate.
  • Volume Scaling: Confusing the 100 cm³ volumetric flask volume with the 10.0 cm³ pipette aliquot volume.
  • Ratio Neglect: Forgetting to multiply by 2 for the 1:2 stoichiometric ratio between Na₂CO₃ and HCl.
🎯 Mark Breakdown (5 marks): 1 mark for correct titres, 1 mark for identifying 3 concordant ticks and correct mean, 1 mark for moles of carbonate (both 100cm³ and 10cm³), 1 mark for moles of HCl, 1 mark for final concentration of HCl to appropriate s.f.
Question 7(b) — Indicator Choice

Methyl Orange Colour Change

✅ Correct Answer: C (yellow to orange)

Since sodium carbonate (alkali) is initially in the conical flask and hydrochloric acid is added from the burette, the solution starts alkaline (yellow) and turns neutral/acidic at the end-point (orange/red).

❌ Why other options fail

  • A (orange to yellow): This is the reverse colour change (going from acid to alkali).
  • B (red to orange): Describes the reverse titration setup (acid in flask, alkali in burette).
  • D (yellow to red): Going all the way to red means you have severely overshot the end-point into excess acid.
🎯 Mark Breakdown (1 mark): 1 mark for selecting option C.
Question 7(c) — Practical Technique

Ensuring End-Point Accuracy

🧠 Exam Technique: Actionable Points

Examiners award marks for precise, active laboratory phrasing. You must state what action is taken and why it improves accuracy.

  • Action 1: Add acid drop by drop near the end-point to avoid overshooting the volume required.
  • Action 2: Swirl/agitate the conical flask continuously to ensure a homogeneous mixture and immediate reaction.
  • Action 3: Rinse sides of the flask with distilled water to wash all reactants into the mixture so none remain stranded on the walls.
  • Alternative Action: Use a white tile underneath the flask to clearly see the colour change.

❌ Examiner Warnings (Where Marks Are Lost)

  • Do not write vague statements like "make the reading more precise" — this is already stated in the stem of the question.
  • Do not suggest reading the meniscus to the bottom or ensuring the burette is vertical here; these affect initial setup, not avoiding overshooting right at the end-point.
🎯 Mark Breakdown (3 marks): 1 mark each for any 3 valid, distinct actions paired with their correct practical reasoning.

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.