Edexcel A-Level Chemistry AS Paper 2, June 2019: Question 6

✅ Correct Answer

Pipette (specifically a volumetric or graduated/glass pipette).

❌ Common Errors

Students frequently confuse volumetric equipment and write burette or measuring cylinder . Measuring cylinders are insufficiently precise for an aliquot in accurate titrations.

✅ Correct Answer

The sodium hydroxide solution ( NaOH(aq) ) or aqueous sodium hydroxide.

💡 Key Knowledge

Burettes must always be rinsed with the solution that will be placed inside them to avoid dilution by residual water droplets, which would alter the titre value.

❌ Common Errors

Writing just "sodium hydroxide" without specifying it is the aqueous solution being put into the burette, or stating water alone.

✅ Correct Answer

C – from colourless to pink

💡 Key Knowledge

Phenolphthalein is colourless in acidic solutions (in the conical flask containing the oxidised wine/ethanoic acid) and pink in alkaline solutions (from the burette).

❌ Common Errors

Choosing methyl orange indicators or reversing the colour change direction (pink to colourless), which applies when titrating an acid into an alkali.

✅ Correct Answer

Titration results that are within 0.20 cm³ of each other.

🧠 Exam Technique

Make sure to learn specific numerical definitions required by exam boards. "Similar" or "close" are insufficient; you must state the exact threshold range ( ± 0.20 cm³ ).

📐 Step-by-Step Calculation

1. Identify concordant titres and calculate the mean:
   Titres given: 27.90, 26.75, 26.85.
   Discard 27.90 (anomalous).
   Mean titre = (26.75 + 26.85) / 2 = 26.80 cm³
2. Calculate moles of NaOH added:
   Moles = (26.80 / 1000) × 0.235 = 0.006298 mol (or 6.298 × 10⁻³ mol)
3. Use the stoichiometric ratio:
   Equation: CH₃COOH + NaOH → CH₃COO⁻Na⁺ + H₂O
   Ratio NaOH : CH₃COOH is 1 : 1, so moles of ethanoic acid in 25 cm³ sample = 0.006298 mol
4. Calculate concentration or scale up original moles:
   Original moles of ethanol in 25 cm³ = (25 / 1000) × 2.50 = 0.0625 mol
5. Calculate percentage oxidised & apply significant figures:
   Percentage = (0.006298 / 0.0625) × 100 = 10.1% (or 10%)
   Rule: Final answer must be to no more than 3 significant figures.

❌ Common Calculation Traps

• Including the anomalous 27.90 cm³ value when calculating the mean titre.
• Forgetting to divide volumes by 1000 when converting cm³ to dm³.
• Failing to round the final answer to 3 significant figures maximum, which loses the final accuracy mark.

✅ Correct Answer

The deep red colour of red wine would obscure or mask the colour change of the phenolphthalein indicator.

🧠 Exam Technique

Consider visual interferences in practical chemistry. If a solution is already intensely coloured, subtle indicator transitions (like colourless to faint pink) cannot be spotted.

📐 Step-by-Step Calculation

1. Find the mass of ethanol in 1 dm³ of gin:
   Concentration = 7.50 mol dm⁻³.
   Mr of ethanol ( C₂H₅OH ) = (2 × 12.0) + (6 × 1.0) + 16.0 = 46.0 g mol⁻¹ .
   Mass = 7.50 × 46.0 = 345 g dm⁻³ .
2. Calculate the volume of ethanol per dm³ using density:
   Density = 0.79 g cm⁻³ (which equals 790 g dm⁻³).
   Volume = Mass / Density = 345 / 0.79 = 436.7 cm³ (in 1000 cm³ total solution).
3. Calculate ABV percentage and match the brand:
   ABV = (436.7 / 1000) × 100 = 43.67% .
   Looking at the table, 43.67% rounds closest to 44%, corresponding to Brand C.

💡 Top-Level Success

To secure all 3 marks, candidates must successfully chain concentration, molar mass, density conversions, and apply the percentage formula cleanly. Allowances are made for minor rounding differences provided clear working is shown.