Edexcel A-Level Chemistry AS Paper 2, June 2018: Question 7
11 marks · Medium difficulty · Calculations
Calculate the concentration of sodium hydroxide from titration data involving ethanedioic acid, select an appropriate indicator, calculate percentage uncertainties in equipment measurements, and determine factors affecting titration uncertainty.
Practise this questionQuestion
Question text
7 This question is about the titration of a weak acid with a strong base.
(a) A standard solution of ethanedioic acid, which is a weak, diprotic acid, can be
used to determine the concentration of a sodium hydroxide solution. 25.0 cm3 of
the ethanedioic acid solution, with concentration 3.80 g dm−3, was pipetted into a
conical flask. A few drops of indicator solution were added. The ethanedioic acid
was titrated with the sodium hydroxide solution which was in the burette. The
titration was repeated and the following results were obtained.
[Molar mass of ethanedioic acid = 90.0 g mol−1]
Titration 1 Titration 2 Titration 3 Titration 4
Final reading / cm3 18.00 17.60 35.30 27.70
Initial reading / cm3 0.00 0.00 17.60 10.05
Titre / cm3 18.00 17.60 17.70 17.65
Titre used to find the
mean titre ( )
Mean titre / cm3
(i) In the appropriate row, tick ( ) those titre values that should be used to find
the mean, and use these titres to calculate it.
Write the value of the mean titre in the box provided in the table of results.
(2)
(ii) Ethanedioic acid is a weak acid. Name a suitable indicator for this titration
and state the colour change at the end-point.
(2)
Name of indicator …
Colour change at the end-point from … to …
(iii) The equation for the reaction of ethanedioic acid with sodium hydroxide is
C2H2O4 + 2NaOH → C2O4Na2 + 2H2O
Calculate the concentration of the sodium hydroxide solution, in mol dm−3.
Give your answer to three significant figures.
(4)
*P51460RA01824*
(b) The uncertainty in each burette reading is ± 0.05 cm3. The uncertainty in the
pipette volume is ± 0.06 cm3.
(i) Calculate the percentage uncertainties for titre 4, and the pipette volume.
(2)
(ii) Which of the following changes would halve the percentage uncertainty in
the volume of liquid measured by the burette?
(1)
A halve the acid concentration and halve the acid volume
B double the acid concentration and leave the acid volume unchanged
C double the acid concentration and halve the acid volume
D halve the acid concentration and leave the acid volume unchanged
(Total for Question 7 = 11 marks)
Mark scheme
Show the mark scheme
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(i) ticks under titration numbers 2, 3, 4 (1) ignore X under Titration 1
example of calculation
17.65 (cm3) (1) 17.60 + 17.70 + 17.65
3 = 17.65
scroll down as mean titre value may be written
below (i) rather than in the table
units not required
must be 2 dp
TE from M1
if Titration 1 has been ticked (17.74) (2)
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(ii) M2 depends on M1
Phenolphthalein/ methyl orange (1) allow any indicator other than litmus or
universal indicator
allow minor errors in spelling of
phenolphthalein but not phenyl….
colourless to pink / red to orange (1) do not award red/pink-red for phenolphthalein
nor yellow for methyl orange
allow correct colour change for other indicators (2)
Question Acceptable Answer Additional Guidance Mark
Number
7(a)(iii) Example of calculation
converts [acid] from g dm-3 to mol dm-3 (1) 3.80/90.0 = *4.22 x 10-2 (mol dm-3)
calculates moles of acid in 25 cm3 (1) ans to M1 x 25 x 10-3
25 x 10-3 x *4.22 x 10-2 = **1.0556 x 10-3
(mol)
allow M1 and M2 in any order
one mark only if not divided by 90.0
calculates moles of sodium hydroxide in titre
cm3 (1) ans to M2** x 2
= 1.0556 x 10-3 x 2 = ***2.111 x 10-3 (mol)
converts moles of sodium hydroxide in titre
to mol dm-3 and gives the answer 3 SF (1) = ans to M3*** x 1000/17.65 = 0.1196
= 0.120 (mol dm-3)
correct answer with no working scores 4 marks (4)
Question Acceptable Answer Additional Guidance Mark
Number
7(b)(i) Example of calculations
burette uncertainty (1) 0.05 x 2 x 100/ 17.65 = (±)0.567/0.57/0.6(%)
pipette uncertainty (1) 0.06 x 100/25 = (±)0.24/0.2(%)
ignore addition of the two uncertainties
ignore SF (2)
Question Acceptable Answer Mark
Number
7(b)(ii) The only correct answer is B
A is not correct because the volume of NaOH needed is divided by 4, uncertainty is x4
C is not correct because moles of acid is the same and uncertainty is the same.
D is not correct because moles of acid halved and uncertainty doubled. (1)
(Total for Question 7 = 11 marks)
How to answer it
Titration Calculations and Experimental Uncertainties Study Guide
What this question tests
This question assesses core practical chemistry and stoichiometry skills, specifically: identifying concordant titration results to calculate a mean titre, selecting appropriate indicators for weak acid-strong base titrations, performing multi-step concentration and mole calculations, calculating percentage uncertainties in apparatus, and understanding how changes to experimental parameters affect measurement uncertainties.
Processing Titration Data
✅ Correct Answer
- Ticks under titration numbers: 2 , 3 , and 4 .
- Mean titre value: 17.65 cm³
🧠 Exam Technique
Concordant titres are values that lie within 0.10 cm³ of each other. Titrations 2 ( 17.60 ), 3 ( 17.70 ), and 4 ( 17.65 ) are concordant. Titration 1 ( 18.00 ) is a rough titre and must be excluded.
❌ Common Errors
Including the rough first titre in the mean calculation will drag the average up and lose you both marks. Ensure you only average the ticked concordant values.
Choosing an Indicator and Colour Changes
✅ Correct Answer
- Indicator: Phenolphthalein (or methyl orange)
- Colour change (Phenolphthalein): Colourless to pink
- Colour change (Methyl orange): Red to orange
💡 Key Knowledge
Ethanedioic acid is a weak acid and sodium hydroxide is a strong base. The equivalence point of this titration falls in the alkaline pH range (~pH 8–10). Therefore, phenolphthalein is ideal. Universal indicator and litmus must never be used in titrations because their colour changes are too gradual.
Calculating Sodium Hydroxide Concentration
📐 Step-by-Step Calculation
- Convert acid concentration from g dm⁻³ to mol dm⁻³:
Concentration in mol dm⁻³ = 3.80 / 90.0 = 0.0422 mol dm⁻³ - Calculate moles of ethanedioic acid in the pipette volume (25.0 cm³):<
Moles = (0.0422 × 25.0) / 1000 = 1.056 × 10⁻³ mol - Use the balanced equation stoichiometry to find moles of NaOH:
Equation: C₂H₂O₄ + 2NaOH → C₂O₄Na₂ + 2H₂O
Since the ratio is 1 : 2 , multiply moles of acid by 2:
Moles of NaOH = 1.056 × 10⁻³ × 2 = 2.111 × 10⁻³ mol - Calculate concentration of NaOH in mol dm⁻³ (using mean titre 17.65 cm³):
Concentration = (2.111 × 10⁻³ / 17.65) × 1000 = 0.1196 mol dm⁻³
Rounding to 3 significant figures: 0.120 mol dm⁻³
❌ Common Calculation Traps
Forgetting the 1 : 2 reacting ratio from the equation is the most frequent place students drop marks. Always check the stoichiometric coefficients before scaling moles!
🧠 Significant Figures
Ensure your final answer is given to exactly 3 significant figures as requested in the stem ( 0.120 , not 0.12 ).
Calculating Experimental Uncertainties
📐 Burette Uncertainty (Titre 4)
A burette reading involves two readings (initial and final), meaning the uncertainty is doubled ( ±0.05 × 2 = ±0.10 cm³ ).
Percentage uncertainty = (0.10 / 17.65) × 100 = 0.567% (Accept 0.57% or 0.6% )
📐 Pipette Uncertainty
A pipette is filled only once to a single calibration line, so uncertainty is not doubled.
Percentage uncertainty = (0.06 / 25.0) × 100 = 0.24% (Accept 0.2% )
❌ Common Errors
Students frequently forget to multiply the burette uncertainty by 2 . Remember: burettes use two readings (start and end), whereas volumetric pipettes and volumetric flasks use only one measurement line.
Halving Burette Percentage Uncertainty
✅ Correct Answer: Option B
Double the acid concentration and leave the acid volume unchanged.
💡 Examiner Commentary & Explanation
Percentage uncertainty is calculated as: (Absolute Uncertainty / Measured Value) × 100 .
To halve the percentage uncertainty, you must double the titre volume measured by the burette while keeping the absolute instrument error constant ( 0.10 cm³ ). Doubling the concentration of the acid in the conical flask requires double the volume of NaOH from the burette to neutralise it, effectively doubling the titre value and halving the percentage uncertainty.
Topics
Physical Chemistry · Core Practicals · Topic 5: Formulae, Equations and Amounts of Substance · Topic 12: Acid-base Equilibria · 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 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.