Edexcel A-Level Chemistry Paper 2, November 2021: Question 8

10 marks · Medium difficulty · Calculations

Calculate the empirical formula and molar mass of a dicarboxylic acid Y from combustion and titration data, and deduce its structure and reduction reagent.

Practise this question

Question

Question 8 features three parts about a dicarboxylic acid Y containing carbon, hydrogen, and oxygen. Part (a) asks to calculate the empirical formula of Y given 1.98 g of Y burns to form 2.51 g of CO2 and 0.69 g of H2O (4 marks). Part (b) gives titration data where 4.34 g of Y made up to 250 cm3 requires 26.10 cm3 of 0.320 mol dm-3 NaOH in a 25.0 cm3 portion, and asks to calculate the molar mass of Y and deduce its structure (5 marks). Part (c) is a multiple-choice question asking which reagent is used to convert a dicarboxylic acid into a diol, with options A (LiAlH4 and ether), B (KMnO4 and H2SO4), C (Sn and HCl), and D (Na2Cr2O7 and H2SO4) (1 mark).
Question text

8 This question is about a dicarboxylic acid Y which is present in some citrus fruits.

Y contains only the elements carbon, hydrogen and oxygen.

(a) A sample of Y with a mass of 1.98g was burned completely in excess oxygen.

The reaction formed 2.51g of carbon dioxide, CO2, and 0.69g of water, H2O.

Use these data to calculate the empirical formula of Y.

(4)

(b) A solution was prepared using 4.34g of the dicarboxylic acid Y made up to a

volume of 250 cm3 with distilled water.

A 25.0 cm3 sample of this solution was then titrated using sodium hydroxide solution,

NaOH(aq), of concentration 0.320 mol dm−3.

The mean titre of sodium hydroxide solution was 26.10 cm3.

Calculate the molar mass of Y using the titration data, and hence

deduce its structure. You must show your working.

(5)

(c) Which of these is used to convert a dicarboxylic acid into a diol?

(1)

A LiAlH4 and ether

B KMnO4 and H2SO4

C Sn and HCl 15

D Na2Cr2O7and H*P65464A01528*2SO4

(Total for Question 8 = 10 marks)

Mark scheme

Show the mark scheme The mark scheme provides detailed step-by-step guidance for question 8. For (a), it details calculations for masses of C, H, and O, moles, and the empirical formula yielding C3H4O4 (4 marks). For (b), it outlines calculations for moles of NaOH, moles of Y in 25 cm3 and 250 cm3, molar mass, and structural deduction of malic acid or similar dicarboxylic acid (5 marks). For (c), it awards 1 mark for option A, explaining why other options are incorrect reducing or oxidising agents.

Question Mark

Answer Additional Guidance

Number

8(a) Example of calculation (4)

• calculation of mass of C and H in CO2 and H2O (1) 0.68455 g C and 0.076667 g H

• calculation of mass of O in acid (1) 1.21878 g of O

• calculation of moles of C, H and O (1) Amount of C = 0.68455/12 = 0.05705 mol

Amount of H = 0.076667/1 = 0.076667 mol

Amount of O = 1.21878/16 = 0.076174 mol

• deduction of empirical formula (1) 1 : 1.34 : 1.34 = 3:4:4 C3H4O4

Allow TE throughout

Ignore SF

Ignore minor rounding errors e.g. 0.684 g of C

NOTE do not award

(2.51 + 0.69) – 1.98 = 1.22 g for M2 as this is

molecular oxygen from combustion NOT

oxygen from compound Y

Question Answer Additional Guidance Mark

Number

8(b) Example of calculation (5)

• calculation of amount of sodium hydroxide (1) 26.10/1000 x 0.320 = 8.352 x 10–3 (mol)

3 8.352 x 10–3 / 2 = 4.176 x 10–3 (mol)

• calculation of amount of Y in 25.0 cm (1)

3 4.176 x 10–3 x (250/25) = 4.176 x 10–2 (mol)

• calculation of amount of Y in 250 cm (1)

4.34 / 4.176 x 10–2 = 103.9 / 104 (g mol–1)

• calculation of molar mass of Y (1)

• deduction of structure of Y (1)

Allow structural / displayed / skeletal or any

combination

Allow TE throughout M1-M4

Penalise 1 SF in M1

Question

Answer Mark

Number

8(c) The only correct answer is A (LiAlH4 and ether) (1)

B is incorrect as acidified KMnO4 is an oxidising agent

C is incorrect as Sn/HCl is too mild a reducing agent

D is incorrect as acidified Na2Cr2O7 is an oxidising agent

(Total Question 8 = 10 marks)

How to answer it

Analysis of Dicarboxylic Acid Y

What this question tests

This multi-step organic analysis question assesses core quantitative chemistry and functional group chemistry. It tests combustion analysis to find empirical formulas, volumetric titration calculations to determine molar mass and deduce structures of dicarboxylic acids, and knowledge of powerful reducing agents in organic synthesis.

Question 8 (a) — Empirical Formula Calculation

Determining Formula from Combustion Data

📐 Step-by-Step Calculation

  1. Find mass of C in CO₂: 2.51 g × (12.0 / 44.0) = 0.68455 g
  2. Find mass of H in H₂O: 0.69 g × (2.0 / 18.0) = 0.076667 g
  3. Find mass of O by subtraction: 1.98 g (total) - (0.68455 + 0.076667) = 1.21878 g
  4. Calculate moles of each element:
    C: 0.68455 / 12.0 = 0.05705 mol
    H: 0.076667 / 1.0 = 0.076667 mol
    O: 1.21878 / 16.0 = 0.076174 mol
  5. Find simplest whole-number ratio:
    Divide by smallest (0.05705):
    C = 1.0, H = 1.34, O = 1.34
    Multiply by 3 to get integers: C₃H₄O₄

❌ Common Calculation Traps

  • The Oxygen Trap: Never calculate the mass of oxygen by doing mass of products minus mass of sample (e.g., 2.51 + 0.69 - 1.98). The extra mass comes from atmospheric oxygen gas used in combustion, not from compound Y!
  • Rounding too early: Keep full calculator values until the final ratio step to prevent rounding errors snowballing.
Mark Breakdown (4 marks): (1) Masses of C and H calculated correctly | (2) Mass of O calculated correctly using the sample mass | (3) Moles of C, H, and O calculated | (4) Correct empirical formula deduced (C₃H₄O₄).
Question 8 (b) — Titration & Structure Deduction

Molar Mass & Structural Determination

📐 Step-by-Step Calculation

  1. Moles of NaOH used: (26.10 / 1000) × 0.320 = 8.352 × 10⁻³ mol
  2. Moles of acid Y in 25.0 cm³: Since it's a dicarboxylic acid (reacts 1:2 with NaOH), divide by 2:
    8.352 × 10⁻³ / 2 = 4.176 × 10⁻³ mol
  3. Moles of acid Y in full 250 cm³ solution: Scale up by factor of 10 (250 / 25):
    4.176 × 10⁻³ × 10 = 4.176 × 10⁻² mol
  4. Calculate Molar Mass: M = mass / moles = 4.34 g / 4.176 × 10⁻² mol = 104 g mol⁻¹
  5. Deduce Structure: Empirical formula C₃H₄O₄ has a mass of (3×12) + (4×1) + (4×16) = 104 g mol⁻¹. Therefore, molecular formula equals empirical formula (C₃H₄O₄). Since it is a dicarboxylic acid containing 3 carbons, the structure is propanedioic acid: HOOC-CH₂-COOH .

💡 Key Knowledge & Technique

  • Stoichiometry check: A dicarboxylic acid has two -COOH groups, meaning 1 mole of acid reacts with 2 moles of NaOH. Missing this 1:2 ratio collapses the titration calculation.
  • Acceptable structures: Displayed, skeletal, or structural formulas are accepted as long as connectivity and functional groups are unambiguous.
Mark Breakdown (5 marks): (1) Amount of NaOH calculation | (2) Amount of Y in 25.0 cm³ | (3) Amount of Y in 250 cm³ | (4) Molar mass calculation (~104 g mol⁻¹) | (5) Correct structure of propanedioic acid.
Question 8 (c) — Multiple Choice

Reduction of Carboxylic Acids

✅ Correct Answer: A

LiAlH₄ in dry ether is a powerful reducing agent strong enough to reduce carboxylic acids directly down to primary alcohols (specifically diols, when starting with a dicarboxylic acid).

❌ Why Other Options Fail

  • B & D (KMnO₄ / K₂Cr₂O₇): These are strong oxidising agents, not reducing agents.
  • C (Sn and HCl): This is a mild reducing agent typically used for reducing aromatic nitro groups to amines, far too weak to reduce carboxylic acids.
Mark Breakdown (1 mark): Selecting option A.

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.