Edexcel A-Level Chemistry AS Paper 1, June 2018: Question 3

8 marks · Medium difficulty · Calculations

Determine the concentration of potassium hydroxide from titration data with sulfuric acid, including apparatus selection, indicator end-point colour, and reading burette scales.

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Question

Question text

3 The reaction of sulfuric acid with potassium hydroxide is a neutralisation.

The equation for this reaction is

H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)

A titration was carried out using the following method.

1. Potassium hydroxide solution of unknown concentration was placed in a burette

and the initial reading was recorded.

2. 25.0 cm3 of sulfuric acid solution, concentration 0.0800 mol dm−3, was transferred

to a conical flask.

3. Three drops of phenolphthalein indicator were added to the sulfuric acid.

4. Potassium hydroxide was added from the burette until the solution just changed

colour and then the burette reading was recorded.

5. Repeat titrations were carried out until concordant titres were obtained.

(a) Select the most appropriate piece of apparatus to measure the 25.0 cm3 of

sulfuric acid.

(1)

A burette

B measuring cylinder

C pipette

D volumetric flask

(b) What is the colour of the solution when neutralisation has just occurred?

(1)

A colourless

B orange

C pale pink

D red

find the initial and final burette readings.

(2)

4 32

5 33 7

*P51459A0724*

Initial reading Final reading

Table of results

Titration number Final reading / cm3 Initial reading / cm3 Titration volume / cm3

2 28.05 1.10 26.95

3 37.65 10.20 27.45

4 32.05 5.00 27.05

(ii) The best value for the mean titre of this reaction is

(1)

A 27.00 cm3

B 27.15 cm3

C 27.25 cm3

D 27.30 cm3

(iii) Calculate the concentration, in mol dm−3, of the potassium hydroxide solution,

giving your answer to an appropriate number of significant figures.

(3)

Mark scheme

Show the mark scheme

3(a) The only correct answer is C (1)

A is not correct because a burette is used to measure varied volumes

B is not correct because a measuring cylinder is less precise

D is not correct because a volumetric flask is less precise

Question

Acceptable Answer Mark

Number

3(b) The only correct answer is C (1)

A is not correct because this is the appearance of the solution before the potassium hydroxide is

added

B is not correct because this is the colour that methyl orange would be in neutral solution

D is not correct because this is a colour sometimes given for the end-point which is incorrect,

and it is the colour of phenolphthalein in acidic solution

Question

Acceptable Answer Additional Guidance Mark

Number

3(c)(i) Example of answer (2)

two correct readings to nearest 0.05 (1) 32.35 and 4.60

correct subtraction of two values to 2 d.p. (1) 27.75

Allow TE for M2 on their burette

readings

Question

Acceptable Answer Mark

Number

3(c)(ii) The only correct answer is A (1)

B is not correct because this is the mean of the three values given without the rough value

C is not correct because this is the mean of the last two values

D is not correct because this is the mean of all four including the rough value

Question

Acceptable Answer Additional Guidance Mark

Number

3(c)(iii) Example of calculation (3)

calculates moles of H2SO4 (1) = 0.0800 x 25 = 0.00200 (mol)

1000

calculates moles of KOH (1) = 0.00200 x 2 = 0.00400 (mol)

calculates concentration of KOH to 2/3 SF = 0.00400 x 1000 (= 0.148148148...) (mol dm-3)

(1) 27.00

= 0.148/0.15 (mol dm-3) to 2 or 3 SF

Allow TE on all stages of the calculation

Correct answer with no working scores (3)

(Total for Question 3 = 8 marks)

How to answer it

Neutralisation Titration: Sulfuric Acid and Potassium Hydroxide

📋 What this question tests

This practical and mathematical assessment covers key experimental and quantitative skills for AS Chemistry:

Selecting appropriate laboratory glassware based on volume accuracy and fixed vs. variable delivery.
Predicting indicator colour changes at the end-point of an acid-base titration.
Reading burette scales correctly to two decimal places (ending in .00 or .05 cm³).
Selecting concordant titres (within 0.10 cm³ or 0.20 cm³) to calculate an accurate mean titre.
Executing structured stoichiometric titration calculations ( n = c × V ) using stoichiometry ratios and quoting answers to suitable significant figures.

Part (a) · 1 Mark

Apparatus for Measuring Fixed Volumes

Selecting the most appropriate equipment for 25.0 cm³ H₂SO₄

✅ Correct Answer

C · pipette (specifically a 25.0 cm³ volumetric/bulb pipette)

💡 Key Knowledge

A volumetric pipette is designed specifically to deliver a single, accurately known fixed volume (typically 25.0 cm³ or 10.0 cm³) with high precision.
A burette is designed for variable, dropwise volume delivery.
A measuring cylinder has far greater percentage uncertainty and lower precision.
A volumetric flask is used to make up standard solutions of a large total volume (e.g. 250 cm³), not to transfer small aliquots into a conical flask.

Mark Scheme: 1 mark for Option C only.

Part (b) · 1 Mark

Indicator Colour Change at the End-point

Observing phenolphthalein when adding alkali from a burette to acid

✅ Correct Answer

C · pale pink

🧠 Exam Technique: Context Matters!

Always identify what is in the conical flask and what is being added from the burette:

Flask: H₂SO₄ (acid) + phenolphthalein → initially colourless.
Burette: KOH (alkali) being added.
End-point: As soon as acid is completely neutralised, the first drop of excess alkali turns the solution from colourless to pale pink (persistent for ~30 seconds).

❌ Common Errors

Confusing the end-point colour with red (Option D): Red/dark magenta indicates strongly alkaline conditions (over-titrated).
Confusing phenolphthalein with methyl orange (Option B, which is orange in neutral solution).

Mark Scheme: 1 mark for Option C only.

Part (c)(i) · 2 Marks

Reading Burette Scales & Calculating Titre

Interpreting the scale from meniscus diagrams

✅ Correct Table Values

Final reading: 32.35 cm³
Initial reading: 4.60 cm³
Titration volume (Titre): 27.75 cm³

🧠 Exam Technique: Reading a Burette

Burette numbers increase downwards.
Always read from the bottom of the meniscus at eye level.
All burette readings must be recorded to two decimal places, where the second decimal place is either 0 or 5 .
For initial reading: bottom of meniscus sits directly on the line below 4.5 → 4.60 cm³ .
For final reading: bottom of meniscus lies halfway between 32.3 and 32.4 → 32.35 cm³ .

❌ Common Errors

Reading upwards from the lower whole number: reading 5.40 instead of 4.60, or 33.65 instead of 32.35.
Omitting the trailing zero (e.g. writing 4.6 instead of 4.60) loses the precision mark.

Mark Breakdown:
• 1 Mark: Both burette readings correct to the nearest 0.05 cm³ (32.35 and 4.60).
• 1 Mark: Correct subtraction (Final − Initial) recorded to 2 decimal places (27.75 cm³). TE (Transferred Error) applies for M2 if readings were misread.

Part (c)(ii) · 1 Mark

Selecting Concordant Titres

Determining the mean titre from experimental runs

✅ Correct Answer

A · 27.00 cm³

💡 Key Knowledge: Concordance Rule

Titres to choose from:

Titration 1: 27.75 cm³ (rough run, not concordant)
Titration 2: 26.95 cm³
Titration 3: 27.45 cm³ (too high, outlier)
Titration 4: 27.05 cm³

Only Titration 2 (26.95) and Titration 4 (27.05) are concordant (within 0.10 cm³ of each other):

Mean = (26.95 + 27.05) / 2 = 54.00 / 2 = 27.00 cm³

❌ Distractor Breakdown

B (27.15 cm³): Mean of titrations 2, 3, and 4 (includes non-concordant titre 3).
C (27.25 cm³): Mean of titrations 3 and 4 only.
D (27.30 cm³): Mean of all four titrations (including rough titration 1).

Mark Scheme: 1 mark for Option A only.

Part (c)(iii) · 3 Marks

Titration Calculation

Calculating the unknown concentration of KOH

📐 Step-by-Step Calculation

Step 1: Calculate the amount (in moles) of H₂SO₄ reacted

Use: moles = concentration × volume (dm³)

n(H₂SO₄) = 0.0800 mol dm⁻³ × (25.0 / 1000 dm³) = 0.00200 mol (or 2.00 × 10⁻³ mol)

Step 2: Use the stoichiometric ratio to find moles of KOH

Look at the balanced equation: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

Molar ratio of H₂SO₄ : KOH = 1 : 2

n(KOH) = 0.00200 × 2 = 0.00400 mol (or 4.00 × 10⁻³ mol)

Step 3: Calculate the concentration of KOH

Volume of KOH used = mean titre = 27.00 cm³ = 27.00 / 1000 = 0.02700 dm³

Concentration = moles / volume

c(KOH) = 0.00400 mol / 0.02700 dm³ = 0.148148... mol dm⁻³

To 3 significant figures: 0.148 mol dm⁻³ (allow 2 SF: 0.15 mol dm⁻³ )

❌ Common Traps

Forgetting the 1:2 ratio: Dividing by 2 instead of multiplying by 2 (or ignoring the stoichiometric ratio entirely).
Unit conversion: Forgetting to divide volumes by 1000 to convert cm³ into dm³.
Significant figures: Writing full calculator string (e.g. 0.148148...) without rounding to an appropriate number of SF (2 or 3 SF matches the data provided).

🧠 Top Tip: Transferred Error (TE)

If you selected the incorrect mean titre in part (c)(ii), full consequential marks (TE) are awarded in this step provided your working is chemically and mathematically sound.

Mark Breakdown:
• Mark 1: Moles of H₂SO₄ = 0.00200 mol
• Mark 2: Moles of KOH = 0.00200 × 2 = 0.00400 mol
• Mark 3: Concentration of KOH to 2 or 3 SF = 0.148 mol dm⁻³ (or 0.15 mol dm⁻³).
Note: A correct final answer with no working shown earns all 3 marks.

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.