Edexcel A-Level Chemistry AS Paper 2, June 2016: Question 8

✅ Correct Answer

• Reagents: Potassium dichromate(VI) ( K₂Cr₂O₇ ) AND sulfuric acid ( H₂SO₄ ). (Both name and formula must match if both are given, or use correct ions: Cr₂O₇²⁻ and H⁺ ).
• Condition: Heat under reflux .

❌ Common Errors

• Forgetting to include the acid alongside potassium dichromate.
• Suggesting hydrochloric or nitric acid (only sulfuric acid is suitable as it does not participate in unwanted redox reactions here).
• Omitting "reflux" or stating simple distillation, which would fail to fully oxidise the intermediate groups to carboxylic acids.

💡 Key Knowledge: 4-Step Preparation Method

1. Weighing: Weigh the ethanedioic acid crystals in a suitable container (e.g. a weighing boat) on a balance, and record the exact mass. Accurately transfer and re-weigh (or wash container).
2. Dissolving: Dissolve the solid in a beaker using a smaller volume of distilled/deionised water (never tap water). Stir with a glass rod.
3. Transfer & Washings: Transfer the solution completely into a 250 cm³ volumetric flask . Rinse the beaker and glass rod with distilled water and add washings into the flask.
4. Make up to mark & Mix: Carefully add distilled water dropwise until the bottom of the meniscus sits exactly on the graduation mark. Stopper the flask and invert/swirl to mix thoroughly.

❌ Common Errors

• Mentioning a standard conical flask or beaker instead of a volumetric flask for making up the standard volume.
• Failing to specify distilled or deionised water.
• Forgetting to rinse the weighing container and glass rod, leaving residual solute behind.

✅ Correct Answer

• From colourless to pink (or red).

❌ Common Errors

• Stating the reverse ("pink to colourless"). Remember: the acid (ethanedioic acid) is in the conical flask with phenolphthalein (making it colourless initially), and NaOH is added from the burette until the first permanent pink coloration appears.
• Using forbidden terms like "clear" instead of "colourless".

🧠 Exam Technique & Consequential Marking

If you make a minor arithmetic slip in earlier steps, subsequent steps use Error Carried Forward (TE). Always write down your intermediate answers clearly so examiners can award method marks.

❌ Common Calculation Traps

• Forgetting to divide by 2 for the stoichiometry ratio between acid and alkali.
• Forgetting to multiply by 10 when scaling from the 25 cm³ titration aliquot up to the 250 cm³ flask.

✅ Correct Answer

• Effect on Titre: The titre will be lower.
• Effect on n: The calculated value of n will be higher.

💡 Examiner Logic

• If crystals are damp, they contain extra unbonded water ( H₂O ). Therefore, for a weighed 1.09 g sample, there is actually less pure H₂C₂O₄ present than assumed.
• Less acid in the flask means fewer moles of NaOH are required to neutralise it, resulting in a smaller (lower) titre.
• Because calculated moles of acid come out lower, the apparent Mᵣ calculated is artificially inflated, which shifts the subtraction in the formula to yield a higher value for n.