Edexcel A-Level Chemistry Paper 3, November 2020: Question 5

✅ Correct Answer

Organic structure of sodium benzoate ( C₆H₅COO⁻Na⁺ or showing ionic bond) + H₂O + CO₂

💡 Key Knowledge

Carboxylic acids are strong enough to react with weak bases like NaHCO₃ , releasing carbon dioxide gas. Phenol (the other component in the mixture) is too weakly acidic to react with sodium hydrogencarbonate, allowing separation.

❌ Common Errors

Incorrectly writing the carboxylate sodium bond as O-Na with a covalent bond instead of showing the ionic species, or forgetting state symbols/byproducts.

✅ Correct Answer

Invert the funnel and then open the tap.

🧠 Exam Technique

Always state the two-step action clearly: invert first so that the tap points away from you and others, then open the tap to release accumulated CO₂ gas.

✅ Correct Answer

Water is more dense than ether (the organic layer).

❌ Common Errors

Writing that water is "insoluble" instead of referring explicitly to density differences.

✅ Correct Answer

To remove any soluble impurities (such as dissolved sodium benzoate) that partitioned into the ether layer.

💡 Key Knowledge

Organic solvents can retain small amounts of aqueous solutes. Washing ensures maximum transfer of the desired salt into the aqueous extract.

✅ Correct Answer

1. The benzoate ion is protonated by the hydrochloric acid.
2. Benzoic acid is substantially less soluble in cold water than its sodium salt, causing it to precipitate out.

🧠 Exam Technique

To secure both marks, you must link the chemical change (protonation to form the neutral acid) to the physical property change (decreased aqueous solubility).

✅ Correct Answer

A Büchner flask connected via a side-arm to a vacuum pump/water aspirator, fitted with a Büchner funnel containing flat filter paper.

❌ Common Errors

Drawing a standard conical flask or specifying a "fluted filter paper" (which is used for gravity filtration, not vacuum filtration).

📐 Step-by-Step Calculation

1. Find mass in 50.0 cm³:
   Mass = (1.70 g / 1000 cm³) × 50.0 cm³ = 0.0850 g
2. Find moles of benzoic acid ( C₆H₅COOH ):
   Molar mass = (6×12.0) + (5×1.0) + 12.0 + (16.0×2) + 1.0 = 122.0 g mol⁻¹
   Moles = 0.0850 g / 122.0 g mol⁻¹ = 6.967 × 10⁻⁴ mol
3. Convert moles to number of molecules:
   Molecules = 6.967 × 10⁻⁴ × 6.02 × 10²³ = 4.19 × 10²⁰

❌ Common Calculation Traps

Do not use 1 sf in final answers. Avoid excessive significant figures (6+ SF); keep your final answer to 3 significant figures matching standard data conventions.

✅ Correct Answer

1. The melting temperature range is wider (or "not sharp") compared to a pure sample.
2. The melting temperature is lower than the literature value ( 122 °C ) due to the presence of impurities (such as unreacted phenol or water).

💡 Key Knowledge

Impurities disrupt the regular crystal lattice of a solid, weakening intermolecular forces. This causes melting to occur over a broader range and at a lower temperature than the pure reference value.