Edexcel A-Level Chemistry Paper 2, June 2025: Question 3

2 marks · Easy difficulty · Multiple Choice

Calculate the atom economy for the alkaline hydrolysis of stearin to form sodium stearate, and state the IUPAC name for glycerol.

Practise this question

Question

Question 3 displays the chemical equation for the saponification of stearin: stearin reacts with 3 NaOH to form 3 molecules of sodium stearate and 1 molecule of glycerol. Part (a) provides a table containing molar masses: stearin (890 g mol⁻¹), sodium hydroxide (40 g mol⁻¹), sodium stearate (306 g mol⁻¹), and glycerol (92 g mol⁻¹), asking for the atom economy by mass for sodium stearate production with multiple choice options A (32.9%), B (34.4%), C (90.9%), and D (98.7%). Part (b) asks for the IUPAC name of glycerol with options A (1,2,3-trihydroxypropane), B (propanetriol), C (2,3-dihydroxypropan-1-ol), and D (propane-1,2,3-triol).

Mark scheme

Show the mark scheme Mark scheme for Question 3. For 3(a), the correct answer is C (90.9%), with explanations of distractors A, B, and D based on incorrect stoichiometry. For 3(b), the correct answer is D (propane-1,2,3-triol), with notes explaining why A, B, and C violate standard IUPAC nomenclature rules.

How to answer it

Saponification: Atom Economy & IUPAC Nomenclature

📋 What this question tests

This question assesses fundamental green chemistry calculations and organic systematic naming conventions:

  • Calculating atom economy from stoichiometric chemical equations and molar mass data.
  • Accounting for stoichiometry (balancing coefficients) when computing reactant and product totals.
  • Applying IUPAC nomenclature rules for multi-hydroxy compounds (polyols).
Part (a) • 1 Mark

Atom Economy of Sodium Stearate Production

Hydrolysis of a triglyceride with aqueous sodium hydroxide

✅ Correct Answer

C — 90.9%

Award 1 mark for selecting option C.

💡 Key Knowledge

  • Atom Economy Formula:
    Atom Economy = (Molar mass of desired product × Stoichiometric coefficient) / Total molar mass of all reactants × 100%
  • By conservation of mass, total mass of reactants equals total mass of all products formed in the stoichiometric equation.

📐 Step-by-Step Calculation

Step 1: Write down the balanced equation and molar masses

1 stearin (890 g mol⁻¹) + 3 NaOH (40 g mol⁻¹) → 3 sodium stearate (306 g mol⁻¹) + 1 glycerol (92 g mol⁻¹)

Step 2: Identify the mass of the desired product

  • Desired product = sodium stearate
  • Number of moles in balanced equation = 3
  • Total mass of desired product = 3 × 306 = 918 g mol⁻¹

Step 3: Calculate the total mass of all reactants (or all products)

  • Reactant mass = 890 + (3 × 40) = 890 + 120 = 1010 g mol⁻¹
  • Check via products: 918 + 92 = 1010 g mol⁻¹ (confirms arithmetic)

Step 4: Compute the percentage atom economy

Atom Economy = (918 / 1010) × 100 = 90.891% ≈ 90.9%

❌ Common Errors & Distractor Breakdown

  • Option A (32.9%): Omitting both stoichiometry numbers: using only 1 sodium stearate and 1 NaOH: (306 / (890 + 40)) × 100 = 32.9% .
  • Option B (34.4%): Using 1 sodium stearate divided by stearin alone: (306 / 890) × 100 = 34.4% .
  • Option D (98.7%): Multiplying the product by 3 but forgetting the coefficient 3 for NaOH in the reactants: (918 / (890 + 40)) × 100 = 98.7% .

🧠 Exam Technique

  • Always circle the stoichiometric coefficients in the reaction equation before substituting values into the atom economy equation.
  • Double-check whether the denominator uses total mass of all reactants, not just the organic substrate.
Part (b) • 1 Mark

IUPAC Nomenclature for Glycerol

Systematic naming of a trihydric alcohol

✅ Correct Answer

D — propane-1,2,3-triol

Award 1 mark for selecting option D.

💡 IUPAC Naming Rules for Polyols

  • Principal chain: 3 carbon atoms bonded with single bonds → propane . Note: the final 'e' is preserved before a consonant (e.g., 't' in -triol).
  • Principal functional group: Three –OH groups are the highest priority group, which gives the suffix -triol .
  • Locants: Each carbon carries one –OH group, requiring the numbering 1,2,3- placed immediately before the suffix.

❌ Why the Distractors are Incorrect

  • A (1,2,3-trihydroxypropane): Incorrect because the alcohol functional group is the principal functional group and must be designated by the suffix ( -ol ), not the prefix ( hydroxy- ). The prefix is only used if a higher priority functional group (such as a carboxylic acid) is present.
  • B (propanetriol): Incorrect because it lacks the necessary numerical position locants ( 1,2,3 ).
  • C (2,3-dihydroxypropan-1-ol): Incorrect because identical functional groups must be grouped together using a multiplying prefix ( tri- ) rather than split between prefix and suffix.

🧠 Top Tip for IUPAC Questions

  • Check for the presence of the letter 'e' in alkane diols and triols: it is propane-1,2,3-triol, NOT propan-1,2,3-triol, because 'triol' starts with a consonant.
  • When in doubt about priority, remember that simple alcohols without carboxylic acids, aldehydes, or ketones always end in -ol .

Topics

Physical Chemistry · Organic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.