OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 8

1 mark · Medium difficulty · Multiple Choice

Calculate the mass of paracetamol in milligrams given the number of moles and its chemical structure.

Practise this question

Question

Multiple-choice question showing the chemical structure of paracetamol and asking to find the mass in milligrams of a tablet containing 3.31 x 10^-3 mol of paracetamol, with options A (493 mg), B (497 mg), C (500 mg), and D (506 mg).
Question text

8 The structure of the painkiller, paracetamol, is shown below.

OH

O

N

H

paracetamol

A tablet contains 3.31 × 10–3 mol of paracetamol.

What is the mass of paracetamol in the tablet?

A 493 mg

B 497 mg

C 500 mg

D 506 mg

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is C, with a mark of 1, noting the correct mass is 500 mg.

8 C 1 AO2.2 ALLOW 500 (This is the correct mass)

How to answer it

Calculating Mass from Moles and Molecular Formula

What this question tests

Determining the relative molecular mass (Mr) from a given skeletal structure, applying the relationship between moles, mass, and molar mass ( mass = moles × Mr ), and performing essential unit conversions between grams and milligrams (mg).

Question 8 (Multiple Choice)

Determining the Mass of Paracetamol in a Tablet

✅ Correct Answer: C (500 mg)

Option C is the only correct choice based on accurate molar mass calculation and unit conversion.

💡 Key Knowledge

  • Molecular Formula of Paracetamol: C₈H₉NO₂
  • Core Formula: Mass (g) = Moles × Mr
  • Unit Conversion: 1 g = 1000 mg (multiply grams by 1000 to get milligrams).

🧠 Exam Technique

Always double-check your atom counting from skeletal structures before calculating the Mr. Remember that terminal points and unlabelled junctions on benzene rings and chains represent carbon atoms with their associated hydrogens.

❌ Common Errors

  • Forgetting to multiply by 1000, resulting in an answer in grams (0.5 g) rather than milligrams (500 mg).
  • Miscounting carbon or hydrogen atoms from the skeletal formula, leading to an incorrect Mr value.

📐 Step-by-Step Calculation

  1. Determine the molecular formula of paracetamol from the structure:
    Count carbons (8), hydrogens (9), nitrogen (1), and oxygens (2) → C₈H₉NO₂
  2. Calculate the relative molecular mass (Mr):
    Mr = (8 × 12.0) + (9 × 1.0) + (1 × 14.0) + (2 × 16.0) = 96.0 + 9.0 + 14.0 + 32.0 = 151.0 g mol⁻¹
  3. Calculate the mass in grams:
    Mass = Moles × Mr = 3.31 × 10⁻³ mol × 151.0 g mol⁻¹ = 0.49981 g
  4. Convert the mass into milligrams (mg):
    0.49981 g × 1000 = 499.81 mg , which rounds neatly to 500 mg (Option C).
Examiner Note: Top-level responses quickly verify the Mr and execute the unit conversion fluently. Option C aligns with standard pharmaceutical dosing (e.g., standard paracetamol tablets contain 500 mg of active ingredient).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.